A fundamental step in learning words is the development of an association between a sound pattern and an element in the environment. Here we explore the nature of this associative ability in 12-month-olds, examining whether it is constrained to privilege particular word forms over others. Forty-eight infants were presented with sets of novel English content-like word–object pairings (e.g. fep) or novel English function-like word–object (e.g. iv) pairings until they habituated. Results indicated that infants associated novel content-like words, but not the novel function-like words, with novel objects. These results demonstrate that the mechanism with which basic word–object associations are formed is remarkably sophisticated by the onset of productive language. That is, mere associative pairings are not sufficient to form mappings. Rather the system requires well-formed noun-like words to co-occur with objects in order for the linkages to arise.