This study, part of a series investigating students' use of online tools to assess problem solving, uses eye-tracking hardware and software to explore the effect of problem difficulty and cognitive processes when students solve gas law word problems. Eye movements are indices of cognition; eye-tracking data typically include the location, duration, and sequence of subjects' fixations on the visual representations. Such information is not usually discovered using conventional assessment methods (e.g., written examinations and scores) when measurements of cognitive performance or task difficulty are needed. The results of this study reveal that when compared to successful ones, unsuccessful students spend more time considering their solutions and fixating more on the questions as they attempt to solve the problem. These results demonstrate that eye-tracking is a useful new approach for exploring problem difficulty and student cognitive activities in chemical education research. (Contains 1 table and 3 figures.)