language comparative problems in lower primary school classes. Specifically which categories of errors children make while solving these problems and to determine whether the results are in accordance with Lewis and Mayer's (1987) consistency hypothesis. Participants were 285 first to fourth grade elementary school students. They were tested individually with consistent language and inconsistent language comparative problems. The results show that there are differences in achievement regarding the type of problem and grade, as well as an interaction between these two variables. The students were more successful in solving consistent language problems than inconsistent language problems. This difference was largest for the 1st grade students, and it was smaller in every subsequent grade, although 4th grade students were also more successful in solving consistent language problems. To better understand the reasons for these differences, we analyzed children's errors in solving both kinds of problems. When solving inconsistent language problems, the participants most frequently committed a reversal error, using the opposite arithmetical operation to solve the problem. These results are in accordance with Lewis and Mayer's (1987) consistency hypothesis, which asserts that individuals develop a schema for the consistent language relational statements and because of that use the opposite arithmetical operation in inconsistent language problems.