Purpose: This study explores the hypothesis that vocabulary growth can have 2 types of effects in morphosyntactic development. One is a general effect, where vocabulary growth globally determines utterance complexity, defined in terms of sentence length and rates of subordination. There are also specific effects, where vocabulary size has a selective impact on the acquisition of grammatical markers and where lexicon is a prerequisite for typological convergence. The study compares the differential effects of vocabulary in 2 measures of morphosyntactic development: omissions of object clitic pronouns and definite articles. Method: Correlation analysis and structural equation models were used to analyze the statistical effects of measures of vocabulary and grammatical development in 110 Spanish-speaking monolingual children ages 3-5 years. Results: The data revealed general effects of vocabulary growth on utterance length and subordination rates and on the use of definite determiners and object pronouns. Specific effects of vocabulary growth were identified for object pronouns but not for determiners. Conclusions: The study found support for a 2-dimensional model separating lexicon and syntax and for 2 types of relationships. Vocabulary development generally determines sentence complexity and further evidence for specific effects in object pronoun use.