For a mature reader´s point of view, reading appears to be an instantaneous and unified cognitive process. Under the scrutiny of research, however, this impression is not accurate. From a cognitive psychology perspective, several interactive component processes including word, sentence and text levels form the act of comprehension rather than one single unified process. In 2001, the International Association for the Evaluation of Educational Achievement assessed the reading achievement of fourth grade students in the Progress in International Reading Literacy Study (PIRLS). The framework of this study distinguishes between four different processes of reading comprehension: - to focus on and retrieve explicitly stated information, - to make straightforward inferences, - to interpret and integrate ideas and information, and - to examine and evaluate content, language and textual elements. This paper presents the analysis of the interrelations between these comprehension processes for the German primary school students including some different IRT-models by which the interrelations are examined.