The renewed interest in the dynamic assessment of specific domains has led to reconsideration of this theory and the technique's contribution to the learning-teaching process. In this article, we analyze some elements concerning the internal structure of a dynamic assessment device of processes involved in reading tasks, establishing some of the device's reliability parameters after its experimental application to 60 students with reading disabilities. Our work analyzes the correlations matrix of the different processes, and determines those reading processes that entail greater difficulties for students. In addition, our study establishes the device's predictive validity in relation to the teacher's evaluation of a student's school performance and progress, analyzing to what extent the device offers additional information as compared with static comprehension, personal-social adjustment or IQ tests. The results reveal the significant predictive validity of the device regarding a student's school performance and progress, as well as its incremental validity in relation to static tests. (Contains 8 tables and 1 figure.)