The objective of this study was to evaluate the effect of a dynamic software simulation on the understanding of the kinetic molecular theory by 7th graders. Students in the control group (n = 62) studied a curricular unit that addressed the differences in arrangement and motion of molecules in the three phases of matter. The experimental group (n = 71) studied the same unit combined with a few computer lessons using a software simulation. The results indicate that the students in the experimental group scored significantly higher than those in the control group. Nonetheless, while both groups of students improved their understanding of the kinetic molecular theory, the overall achievements were very low. These findings suggest that the simulation improved the understanding of the 7th graders; however, it was insufficient in itself to promote meaningful learning. Statistically significant gender differences were not observed. This paper concludes with a discussion of the educational implications of this study.