A study investigated the relationship between prior knowledge, age, instruction, and the relative difficulty and importance of a text in the reading processes and knowledge acquisition of elementary school children in Belgium. Two groups of 42 students (10 and 12 year olds) were given a knowledge questionnaire two months prior to the experimental task. For the experimental task, students read a selection and were told to memorize it or summarize it. Results indicated that age, prior knowledge, and relative importance of the text were significant factors in how well the students read (acquisition input). Age, relative importance, and relative difficulty of the text affected memorization (acquisition output). The results suggest that the analysis of knowledge acquisition by text must take into account both input and output performance variables. A 21-item list of references concludes the document. (SRT)