This paper describes a dataset consisting of longitudinal data gathered in the BRXXX project. The aim of the project was to investigate the effectiveness of online mathematics mini-games in enhancing primary school students' multiplicative reasoning ability (multiplication and division). The dataset includes data of 719 students from 35 primary schools in the Netherlands, who were followed from end Grade 1 to end Grade 3. Participating schools were randomly divided over three experimental conditions (playing at school, at home or at home with debriefing at school) and a control group. Tests of multiplicative reasoning ability were administered at the end of Grade 1 (pretest) and the end of Grade 2 and Grade 3 (posttests). In addition to the scores on these tests, the dataset includes measures of gameplay behavior, i.e., the time and effort students put in the games, and some student background variables.