This study investigated whether and when children establish various semantic relations between old and new words. Fifty two-year-olds were taught labels for objects previously referred to by an overextended term. We found that children were more likely to learn a new label when (a) it referred to a new object that was perceptually dissimilar, rather than similar, to a known one, and (b) when linguistic information indicated it had an inclusion, rather than a mutually exclusive, relation to a known label. Children were more likely to interpret a new label as mutually exclusive to a known one when their referents were perceptually dissimilar. These findings are discussed in light of theories of lexical development, particularly with regard to conceptualizations of constraints on the acquisition of word meaning.