Developing readers of English appear to favor phonograms over grapheme-phoneme correspondences (GPCs) to read unknown words. For polysyllabic polymorphemic (PSPM) words, the morphophonemic nature of English means elementary-age children may focus on roots and affixes. Does developing readers' PSPM word reading accuracy relate to the morphological units, the nonmorphological, or both? In this study, 3rd and 4th graders (N = 202) read PSPM words (N = 45), and models were constructed to answer this question. A nonmorphological polysyllabic model showed a main effect of phonological awareness; a Vocabulary Size × Word Frequency interaction, with larger vocabularies improving accuracy for low-frequency words; and a GPC Knowledge × Word Frequency interaction, with a slight negative GPC knowledge effect for all but low-frequency words. A polymorphemic model showed main effects of word-specific root word knowledge, general root word reading, vocabulary, and word frequency. A Morphological Awareness × Morphological Transparency interaction showed morphological awareness affected accuracy for shift words more than transparent ones. In a combined model, effects were the same, except GPC knowledge positively affected accuracy for very low-frequency words, vocabulary and frequency had only main effects, and a bigram frequency effect was found. The polysyllabic model reduced child and item variance by 45% and 43%, respectively; the polymorphemic model by 62% and 67%; and the combined model by 69% and 69%. The results suggest that elementary-age developing readers rely on morphological information to read PSPM words. Implications for models of reading and reading intervention are discussed.