A straightforward prediction from attention restoration theory is that the level of incompatibility in a person's life should be positively correlated with that person's level of mental (or directed attention) fatigue. The authors tested this prediction by developing a new self-report measure of incompatibility in which they attempted to isolate all the six categories of incompatibility described by S. Kaplan: distraction, deficit of information, duty, deception, difficulty, and danger. Factor analysis revealed six factors that corresponded reasonably well to those six categories. This article found that a composite incompatibility score was positively correlated with a separate self-report measure of mental fatigue and so were all six of the subscale scores. With the exception of the score for duty, these positive correlations remained after partialing out a separate measure of the level of stress in the person's life. The authors concluded that the proposed categories of incompatibility can be validly measured; that the constructs of incompatibility, mental fatigue, and stress are discriminable from each other; and that incompatibility is generally positively correlated with mental fatigue. (Contains 3 tables and 1 note.)