There has been considerable debate over the existence of the ‘vocabulary spurt’ phenomenon — an apparent acceleration in word learning that is commonly said to occur in children around the age of 18 months. This paper presents an investigation into modelling the phenomenon using data from almost 1800 children. The results indicate that the acquisition of a receptive/productive lexicon can be quite adequately modelled as a single growth function with an ecologically well founded and cognitively plausible interpretation. Hence it is concluded that there is little evidence for the vocabulary spurt phenomenon as a separable aspect of language acquisition.