We report results from two studies on the underlying dimensions of morphological awareness and vocabulary knowledge in elementary-aged children. In Study 1, 99 fourth-grade students were given multiple measures of morphological awareness and vocabulary. A single factor accounted for individual differences in all morphology and vocabulary assessments. Study 2 extended these results by giving 90 eighth-grade students expanded measures of vocabulary and morphology that assessed (a) definitional knowledge, (b) usage, (c) relational knowledge, and (d) knowledge of morphological variants, with each potential aspect of knowledge assessed using an identical set of 23 words to control for differential knowledge of specific vocabulary items. Results indicated that a single-factor model that encompassed morphological and vocabulary knowledge provided the best fit to the data. Finally, explanatory item response modeling was used to investigate sources of variance in the vocabulary and morphological awareness tasks we administered. Implications for assessment and instruction are discussed.