Prior studies of the relationship between phonological information and grammatical category assignment have focused on whether these relationships exist and whether people have learned them. This study investigates whether these relationships affect preschool children’s vocabulary acquisition in a laboratory setting. Child participants learned 12 vocabulary words (6 nouns and 6 verbs) under three conditions, in which, (1) the syllable number/grammatical category relationship matched English, (2) the syllable number/grammatical category relationship was opposite to English, or (3) there was no relationship between syllable number and grammatical category. In the initial presentation of the words, children assumed that the novel words matched the pattern found in English. When the syllable number/grammatical category pattern matched that of English, the children learned more of the words. Phonological information also predicted error patterns. These results suggest that any account of vocabulary acquisition should consider the role of phonological information.