Abstract: We investigate students’ eye movements when they solve sequential and simultaneous synthesis physics problems. In sequential synthesis problems, multiple events occur chronologically, whereas in simultaneous synthesis problems, multiple events take place concurrently. We captured students’ eye fixations on each problem diagram and recorded their verbal descriptions of problem-solving approaches. Results indicate that for sequential synthesis tasks, students fixated heavily and frequently on regions of a diagram where information is unknown or on empty space related to future events. Further, most students could invoke all relevant principles and intermediate variables to solve the sequential problems. However, for simultaneous synthesis tasks, students prioritized aspects of the diagram where information is given in the problem statement. Moreover, they mostly invoked only one fundamental principle to solve the problems by either using the values given in the statement or identifying one relevant intermediate variable. These findings may be explained by the differences in the spatiotemporal occurrence of the multiple events in the two types of synthesis problem. The chronological events in sequential synthesis tasks seem to support students’ sensemaking, but the concurrent events of simultaneous synthesis tasks do not.