The study investigates measurement properties of the reading assessment tasks used in the IEA 1991 Reading Literacy (RL) study and the Progress in International Readings Study (PIRLS) 2001 study. The analysis is based on data from the Swedish PIRLS study, comprising 16,676 students in grades 3 and 4. As an extension to the basic design, not only the PIRLS tasks were administered, but each student also completed 1 of the 2 booklets from the 1991 study. Using missing-data modeling techniques, confirmatory factor analysis models were estimated and tested for the complete set of reading tasks. Results show that both sets of tasks measure comprehension of reading continuous text, but that each of them also represents unique sources of variance. In the RL instrument, reading speed is one such component, and performance on tasks from the documents domain is another. The PIRLS tasks are influenced by the requirement to produce constructed responses. It is also demonstrated that the shared context among items referring to a particular text is a source of systematic variance. (Contains 9 tables.)