This study used aptitude treatment interaction design to examine how feedback formats (specific vs. holistic) and executive thinking styles (high vs. low) affect web-based peer assessment. An Internet-based (anonymous) peer-assessment system was developed and used by 58 computer science students who submitted assignments for peer review. The results indicated that while students with high executive thinking styles significantly improved over two rounds of peer assessment, low executive students did not improve through the cycles. In addition, high executive students contributed substantially better feedback than their low executive counterparts. In the second round of peer assessment, thinking style and feedback format interactively affected student learning. Low executive students receiving specific feedback significantly outperformed those receiving holistic feedback. In receiving holistic feedback, high executive thinkers outperformed their low executive counterparts. This study suggests that future web-based peer assessment adopts a specific feedback format for all students.