The article deals with the relevant linguistic issue of correlation between word spelling and the distinction of units belonging to different grammatical classes. The concepts of word and part of speech are contrasted. The author has revealed the peculiarities of lexical units functioning in written speech, which enables their part-of-speech status identification. The analysis of criteria, suggested by the linguists for differentiation of homonymous adverbs, preposition-and-case-form combinations, and derivative prepositions resulted in proposing a procedure of consecutive operations, accomplished to identify the part-of-speech status of grammatically homonymous words. The results of the linguistic experiment show how in speech practice native speakers solve the problems of part of speech determination and establishing the spelling of such words. It was revealed that the methods for distinguishing grammatical homonyms used by recipients, in many cases, do not lead to the correct solution. The existing codified guidelines are not applied while writing, as spelling of the major part of grammatically homonymous words does not meet the requirements of the norm. To solve the problem under consideration, it is necessary to adjust the content of spelling rules and change the spelling of a number of words, where traditional spelling principles are reflected.