Recent studies have found correlations between sentence-level tests and reading comprehension. However, the task demands of sentence-level tests are not well understood. The present study investigated syntactic knowledge as a construct by examining the convergent and discriminant validity of two sentence-level tasks, sentence comprehension and sentence repetition, designed to test syntactic knowledge and their relation with reading comprehension. Results from 86 Grade 6 students showed that the syntax tests were more highly correlated with each other than with tests of working memory and vocabulary. This suggests that the syntax measures tap into a set of skills that are at least partially separate from these other cognitive constructs. Furthermore, syntactic knowledge explained unique variance in reading comprehension beyond controls. The syntax tasks were working memory dependent, but working memory was not the primary reason why syntax tasks are correlated with reading comprehension.