The goal of this article is to account for the resolution of vowel sequences across word boundaries in Catalan. Specifically, the paper accounts for the distribution of hiatuses and syllable contraction cases between two lexical words in this language. The article argues that V1 (the last vowel of the first word) does not undergo any change if it is followed by a vowel V2 bearing nuclear stress (or phrasal stress) prominence. The blocking of V1 glide formation will be seen in relation with the systematic maintenance of schwa in this position (canti ara [i »a] ‘you sing.imp now’, canto ara [u »a] ‘I sing now’, tallo ungles [u »u] ‘I cut nails’, canta ara [ »a] ‘he/she sings now’). Blocking of glide formation or schwa deletion is thus not due to rhythmic reasons (stress clash), as some previous studies have contended, but rather to the presence of a nuclear stress prominence on V2. This phenomenon will be interpreted as the instantiation of an alignment constraint which aligns the word-initial nuclear stressed foot to the left edge of the prosodic word. This alignment constraint triggers a ‘prosodic isolation’ phenomenon which prevents vowel gliding or deletion from applying. Finally, the paper also accounts for vowel sandhi in contexts where V2 is not stressed: in these contexts, syllable contraction is the norm. * Earlier versions of this work were presented at PaPI 2003 (Phonetics and Phonology in Iberia, Lisbon), at the Toulouse International Conference “From representations to constraints” (Toulouse, July 2003) and at the XVth International Congress of Phonetic Sciences (Barcelona, August 2003). We are grateful to those who attended these meetings for interesting observations and comments, and especially to Eulalia Bonet, Sonia Colina, Sonia Frota, Jose Ignacio Hualde, Michael Kenstowicz, John Kingston, Maria Rosa Lloret, Joan Mascaro, John McCarthy, Daniel Recasens, Elizabeth Selkirk, Donca Steriade, Hubert Truckenbrodt, Marina Vigario, and Max Wheeler for discussion of some parts of the material included in the article. Thanks are also due to Nuria Riera for transcribing the vowel contacts present in 5 spontaneous conversations of the Corpus Oral de Catala and to Marta Paya and Lluis Payrato for kindly providing us with a copy of this database before its publication. Finally, we thank Teresa Barenys, Julia Cufi, Teresa Espinal, Anna Gavarro, Nuria Marti, Jaume Sola, and Xavier Vall, who patiently responded to our questionnaire. All remaining errors are of course ours. This research was funded by grants 2002XT-00032 and 2001SGR 00150 from the Generalitat de Catalunya and BFF2003-06590 and BFF2003-09453-C02-C02 from the Ministry of Science and Technology of Spain.