The current study attempts to differentiate effects of phonotactic probability (i.e. the likelihood of occurrence of a sound sequence), neighbourhood density (i.e. the number of phonologically similar words), word frequency, and word length on expressive vocabulary development by young children. Naturalistic conversational samples for three children (age 1;4-3;1) were obtained from "CHILDES." In a backward regression analysis, phonotactic probability, neighbourhood density, word frequency, and word length were entered as possible predictors of ages of first production of words for each child. Results showed that the factors affecting first production of words varied across children and across word types. Specifically, word length affected ages of first production for all three children, whereas the other three variables affected only one child each. The implications of these findings for models of expressive vocabulary development are discussed. (Contains 1 footnote.)