Phonological words play a crucial role in phonology, but where exactly they are produced in syntax is not clear. I propose a theory whereby the syntax issues phonological word diacritics to the complex constituents it creates. Additionally, certain morphemes can be specified in the lexicon as possessing these diacritics. The phonology then interprets the diacritics—sometimes it ignores them, and other times it makes phonological words to satisfy language-specific prosodic requirements. The resulting theory is demonstrated on the complex patterning of prepositions in Russian. The class of prepositions in Russian has certain syntactic traits in common, but there are many patterns where prepositions diverge according to their phonological word status. There are correlations between morphosyntactic structure and phonological word status: morphologically complex prepositions are always words. On the other hand, the presence of a morphological root, phonological size, and stress do not align with word status. The large range of phonological and morphosyntactic patterns involving prepositions in Russian demonstrates the need for an explicit and rich theory of word formation at the phonology-syntax interface.