The present paper consists of two studies intended to further the understanding of intolerance of uncertainty and its relationship to worry. The first study examined the psychometric properties of the English version of the Intolerance of Uncertainty Scale (IUS), which has already been validated in French. Factor analysis indicated that the IUS has a 4-factor structure that represents the idea that uncertainty is stressful and upsetting, uncertainty leads to the inability to act, uncertain events are negative and should be avoided, and being uncertain is not fair. The IUS has excellent internal consistency, good test-retest reliability, and convergent and divergent validity when assessed with symptom measures of worry, depression, and anxiety. The second study attempted to assess the unique relationship between intolerance of uncertainty and worry, beyond constructs already associated with worry such as perfectionism and control. Furthermore, the study assessed the distinction between intolerance of uncertainty and intolerance of ambiguity. The results suggest that worry has a stronger relationship with intolerance of uncertainty than perfectionism, control, and intolerance of ambiguity. Moreover, the results indicate that intolerance of uncertainty and intolerance of ambiguity are distinct constructs. Overall, this study suggests that the IUS is a sound measure of intolerance of uncertainty and supports the idea that intolerance of uncertainty is an important construct involved in worry
