In three experiments we tested the effects of spatial visualization ability on performance of a motion-verification task, in which subjects were shown a diagram of a mechanical system and were asked to verify a sentence stating the motion of one of the system components. We propose that this task involves component processes of (1) sentence comprehension, (2) diagram comprehension, (3) text-diagram integration, and (4) mental animation. Subjects with low spatial ability made more errors than did subjects with high spatial ability on this task, and they made more errors on items in which more system components had to be animated to solve the problem. In contrast, the high-spatial subjects were relatively accurate on all trials. These results indicate that spatial visualization is correlated with accuracy on the motion-verification task and suggest that this correlation is primarily due to the mental animation component of the task. Reaction time and eye-fixation data revealed no differences in how the high- and low-spatial subjects decomposed the task. The data of the two groups of subjects were equally consistent with a piecemeal model of mental animation, in which components are animated one by one in order of the causal chain of events in the system.