Listening and reading comprehension of several syntactic structures was analysed with second to fifth grade readers. Three different structures were compared in both reading and listening: simple sentences, simple sentences with morphological markers for number and gender, and complex relative sentences. The difference between reading and listening was analysed systematically. Data showed that this difference changes with school level and with the type of syntactic structure and their specific constraints. The constraints that differentiate most between reading and listening were shown to be the identification of morphological markers necessary for identifying words (for simple sentences with markers) or the handling of working memory resources (for complex sentences).