Reading comprehension is based on the efficient accomplishment of several cognitive processes at the word, sentence, and text level. To the extent that each of these processes contributes to reading comprehension, it can cause reading difficulties if it is deficient. To identify individual sources of reading difficulties, tools are required that allow for a reliable and valid assessment of individual differences in specific cognitive processes of reading comprehension. The present study shows the usefulness of this process-oriented approach to assessing reading comprehension skills by using the example of a test for assessing syntactic skills in German primary schoolchildren. The test comprises a grammaticality judgment task that contains items with carefully varied features that are known to facilitate or impede syntactic processes. By means of explanatory item response models, we demonstrate that empirical item difficulties vary as a function of experimentally varied item features, indicating that the test indeed assesses syntactic skills. Moreover, the test measures individual differences in syntactic skills across the whole range of skill levels and is sensitive to developmental changes. We conclude that the test is a valid tool to assess individual differences and to detect deficits in this component process of reading comprehension.