International comparative research in general and TIMSS research in particular struggle to explain differences in pupil performance across countries, as well as changes in pupil performance over years. This study aims at determining the influence of textbook content on fourth-grader performance in Natural Sciences in the scope of TIMSS research. Content analysis was performed by applying TIMSS 1995 Document Analysis methodology and corresponding framework for Latvian, Kazakhstanian, Russian Federation, Ukrainian and USA textbooks. Each textbook is divided into units, which are further divided into blocks. Under TIMSS 1995 Document Analysis Framework, each block is granted Content, Performance Expectations and Communicating codes (one or more from each group). The content of a textbook is analyzed by code analysis. The study shows that changes in textbook content in Latvia are an important factor determining changes in fourth-grader performance in Science. It also explains the differences in relative performance in the Science Content Domens for Latvia, Kazakhstan and Ukraine. Similarly, the study shows that textbook analysis can be an effective instrument for explaining both trends in student performance and performance differences across countries. This is specially the case for strongly centralized educational systems that have a small number of textbook sets.