Background: Studies of syntactic comprehension and reading comprehension use a wide range of syntactic comprehension tests that vary considerably in format. The goal of this study was to examine to which extent different formats of syntactic comprehension tests measure the same construct. Methods: Sixty-nine Grade 4 students completed multiple tests of decoding, vocabulary, fluid-reasoning skill, reading comprehension and three tests of syntactic comprehension: the TROG-2, a sentence repetition test and a whodunnit test. Results: There was limited shared variance between the syntactic comprehension tests, indicating that they only partially tapped into the same construct. Furthermore, the TROG-2 was more highly correlated with fluid-reasoning skill than the other syntax tests. Finally, the TROG-2 and the whodunnit test explained additional variance in reading comprehension after controls, while sentence repetition did not. Conclusions: There are important differences between syntactic comprehension tests. This has consequences both for the interpretation of previous studies of syntactic comprehension and reading comprehension and for the design of future studies as the choice of syntactic comprehension measure may directly influence the results of the study.