In spite of a large body of research, deep knowledge regarding the effect of computer-based instruction systems (CBI) is lacking. The objective of this research was to investigate the effectiveness of CBI in support of mastery learning. The analysis is based on 333 students who used a CBI system, called management information systems tutor (MIST), for a nine-week period. Results of the analysis show that the MIST helped students improve their performance. In addition, a significant relationship was observed to exist between a student's performance and time on task. However, some students neither reached mastery performance nor did any significant change in their time spent on task occur. This is contrary to the belief held by some researchers in the field of CBI. These results are analyzed with respect to findings from motivational and self-learning research.