Purpose: We examined whether affix type and base word transparency explained variation in third- through sixth-grade students' performance on a number of morphological awareness tasks. Method: Third- through sixth-grade students (n > 500 at each grade) completed morphological awareness tasks from the Morphological Awareness Test for Reading and Spelling, which represent the ways individuals may use their morphological awareness to support reading and spelling. Explanatory item response models were used to understand the role of affix type and base word transparency on students' performance on six morphological awareness tasks. Results: For all grades, 73%-83% of variance in students' performance was due to differences across individual items. Furthermore, when task effects, affix type, and base word transparency were included simultaneously in the model, affix type was not a significant predictor; there was a significant effect of base word transparency and task. Specifically, the probability of a correct response was greater on task items in which inflected or derived words were transparent with their base word (e.g., friend > friendly) compared to items in which there was a shift in both the phonological and orthographic aspects of the base word (e.g., attend > attention). Conclusions: These findings emphasize the importance of considering base word transparency when assessing students' morphological awareness skills with less emphasis on affix type, at least for third- through sixth-grade students. Our results also point to the importance of administering a variety of morphological awareness tasks to fully capture an individual's morphological awareness skills. Collectively, researchers and practitioners should ensure assessment items on multiple measures of morphological awareness vary in their base word transparency to potentially capture a range of student performances. [For the corresponding grantee submission, see ED624675.]