Learning involves complex cognitive processes such as perception, attention, and memory, with reading being a common method to acquire knowledge. Textbooks are often supplemented with additional materials such as self-test questions and demonstration tasks to enhance learning. However, the impact of these supplemental materials, especially when they interrupt reading, is not well understood. This study examined how interruptions by supplemental materials affect reading comprehension and learning outcomes. In three experiments, participants read passages on cognitive psychology topics with supplemental materials in the form of demonstration tasks or additional text placed either before entire passages (uninterrupted condition) or in the middle of the passages (interrupted condition). Experiment 1 found that interruptions led to lower accuracy on test questions and longer reading times. In Experiment 2, the duration of interruptions was increased, but this did not result in significant effects on test question accuracy, although reading times were longer for interrupted passages. Experiment 3 used thematically related supplemental text as interruptions, which also did not affect test question accuracy but led to longer reading times. The consistent finding across all experiments was that participants read the second halves of passages faster than the first halves, suggesting that their mental model of the text facilitated understanding as they progressed. The results indicate that the placement of supplemental materials within short passages might not significantly impact learning outcomes, although interruptions do affect reading times. These findings challenge the importance of the location of supplemental materials in textbooks and suggest that strategic placement might not be as critical as previously thought. Future research should explore longer passages, more frequent interruptions, and different types of supplemental materials to better understand their effects on learning. Aligning textbook content with evidence- based principles can help educators and instructional designers optimize learning environments and improve student engagement and comprehension. [The dissertation citations contained here are published with the permission of ProQuest LLC. Further reproduction is prohibited without permission. Copies of dissertations may be obtained by Telephone (800) 1-800-521-0600. Web page: http://bibliotheek.ehb.be:2222/en-US/products/dissertations/individuals.shtml.]