The present study examined whether syntactic comprehension constitutes a source of individual differences separate from vocabulary and of relevance to reading comprehension. One hundred and sixty-one Danish Grade 6 students completed multiple tests of syntactic comprehension, vocabulary, decoding, and reading comprehension. Syntax measures were designed to isolate syntactic skills. Syntax and vocabulary measures included expressive and receptive tasks to balance method variance. Structural equation models supported that syntactic comprehension and vocabulary constituted separate, moderately correlated factors, and that syntactic comprehension had the strongest direct effect on reading comprehension. The results suggest that syntactic comprehension and vocabulary are separate sources of individual differences, and thus that some students misunderstand texts due to difficulties establishing how individual words relate to each other in sentences based on syntactic signals. The study also suggests that for sixth-grade students, individual differences in syntactic comprehension are at least as important as individual differences in vocabulary.