ABSTRACTParent report instruments adapted from the MacArthur-Bates Communicative Development Inventories (CDI) examined vocabulary development in children aged 0 ; 8 to 2 ; 6 for two Chinese languages, Mandarin (n=1694) and Cantonese (n=1625). Parental reports suggested higher overall scores for Mandarin- than for Cantonese-speaking children from approximately 1 ; 4 onward. Factors relevant to the difference were only-child status, monolingual households and caregiver education. In addition to the comparison of vocabulary scores overall, the development of noun classifiers, grammatical function words common to the two languages, was assessed both in terms of the age and the vocabulary size at which these terms are acquired. Whereas age-based developmental trajectories again showed an advantage for Beijing children, Hong Kong children used classifiers when they had smaller vocabularies, reflecting the higher frequencies and greater precision of classifier use in adult Cantonese. The data speak to the importance of using not just age, but also vocabulary size, as a metric by which the acquisition of particular linguistic elements can be examined across languages.