Abstract This study explores the relevance of suffix allomorphy for processing complex words. The question is whether structural invariance of the morphological category (i.e., lack of allomorphy) would affect the processing of Finnish derived words. A series of four visual lexical decision experiments in which alternatively surface and base frequency was manipulated showed that the two invariant suffixes, namely denominal –stO and deadjectival –hkO, showed reliable effects of base frequency, whereas for the two categories with suffix allomorphy, deverbal –Us and deadjectival –(U)Us, only surface frequency played a role. A further experiment showed that even with the most frequent variant of –(U)Us, namely –Ude-, response latencies were a function of surface frequency only. It is shown that neither the results from the experiments here nor previous findings from processing Finnish words can be accounted for by suffix frequency, the frequency ratio between the derived word and its base, or morphological productivity in any straightforward manner. We conclude that the lack of allomorphy, that is, structural invariance, significantly adds to affixal salience and therefore enhances morphological decomposition. The implications of this finding for models of lexical processing are discussed. We would like to thank Matti Laine and an anonymous reviewer for their helpful comments. Notes 1To be precise, typologically Finnish morphology is fusional-agglutinative in distinction to that of Turkish, which gives us a schoolbook example of agglutinative morphology. 2The capital vowels in –jA and the suffixes investigated here, namely –stO, –hkO, – (U)Us, and –Us, refer to the archiphoneme marking phonological adjustment following from Finnish vowel harmony. That is, for instance in the case of –jA, the suffix is realised as either /ja/ or /jæ/ depending on whether the stem has a back or front vowel respectively. 3Although perceptually homonymous with the deverbal –jA, the particular variant of partitive plural inflection is structurally bimorphemic with a combination of a partitive allomorph –A and a phonologically motivated change of the plural marker –i to –j in /V_V/ contexts. 4The frequency and productivity counts are based on the lexical database compiled from seven consecutive annual volumes of a Finnish newspaper Karjalainen (1991–1997). The material consists of 34.5 million word tokens and covers a reasonably long stretch of time to provide a representative view of the use and productivity of derivational affixation. The corpus (Karjalaisen korpus, 34.5 million-word token computer-based newspaper corpus of Finnish based on Karjalainen (Joensuu), compiled by J. Niemi and his associates at the Linguistics Department, University of Joensuu, SGML form created at the Department of General Linguistics, University of Helsinki) is available from Kielipankki at http://www.csc.fi/kielipankki/. 5This is also reflected in the fact that in the Karjalainen database –(U)Us is attested in noun bases with about 120 types and 12 hapaxes, resulting in a comparably low index of productivity, p =.0001 (cf. Table 1). 6The results from another run of the surface frequency experiment with different items (16 per condition) and 25 participants showed a significant difference between the High Surface (674 ms) and Low Surface (722 ms) conditions, t 1(24) = 4.05, p <.001, t 2(30) = 2.46, p <.05, confirming that surface frequency is indeed a significant factor in the processing of words in –hkO. 7In line with the tradition in terminology, we have also here used the term Surface Frequency to reflect the cumulative frequency of the word and all its inflected forms. It should be noted though that the surface frequency proper (here the genitive form) highly correlates with the surface frequency as we defined it. That is, for the base frequency experiment, the genitive frequencies are matched (High: 0.2; Low: 0.3; t<1), for the surface frequency experiment they are manipulated (High: 4.3; Low: 0.3; p<.001). 8Although they do note that the connectionist three-layer network model of Davis, van Casteren, and Marslen-Wilson (Citation2003) can also accommodate their pattern of findings. 9More precisely, –Us and –(U)Us are homonymous only in nominative singular and in all plural cases except the nominative. Furthermore, words in deadjectival –(U)Us quite rarely occur in any of the plurals, where the overlap of the two suffixes would be greatest. For example, for a frequent noun in –(U)Us, 'kauneus' (beauty), that is found 722 times in the Karjalainen database, there is not a single instance of plural inflection. 10Also, Finnish stem allomorphy is abundant and often not amenable to straightforward rule-based generalisations. It might therefore come as no surprise, that, in contrast to the wealth of experimental results supporting the phonological underspecification account for the representation of English stem allomorphy, several studies have found evidence for the position that in Finnish stem allomorphs have separate representations in the mental lexicon. This evidence is obtained from a variety of experimental paradigms, such as lexical decision (e.g., Laine, Vainio, & Hyönä, Citation1999), priming (Järvikivi & Niemi, Citation2002a), masked priming (Järvikivi & Niemi, Citation2002b), slips of the tongue (Niemi & Laine, Citation1992), and aphasia studies (Laine et al, Citation1995; Laine & Niemi, Citation1997). Additional informationNotes on contributorsJuhani Järvikivi Correspondence should be addressed to Juhani Järvikivi, Department of Psychology, Assistentinkatu 7, FIN-20014 University of Turku, Finland. juhani.jarvikivi@utu.fi