Intolerance of uncertainty (IU) is related to anxiety, depression, worry, and anxiety sensitivity. Precedent IU measures were criticized for psychometric instability and redundancy; alternative measures include the novel 45-item measure (Intolerance of Uncertainty Index; IUI). The IUI was developed in French with 2 parts, assessing general unacceptability of uncertainty (15 items, Part A) and manifestations of uncertainty approximating more common anxiety disorder symptoms (30 items, Part B). The psychometric stability of the back-translated English items of the IUI as well as the incremental variance of Parts A and B remain to be assessed. The current study involved 2 samples of English-speaking community participants (n = 437 and n = 309; 73% women and 27% men) who completed the IUI and several related measures. Exploratory and confirmatory factor analyses suggested a refinement of IUI items as well as a unitary structure for Part A and a 3-factor structure for Part B. Regression results suggested Parts A and B each provide incremental validity in measures of worry, generalized anxiety disorder symptoms, negative problem orientation, and depression. Comprehensive results, implications, and future research directions are discussed. (Contains 6 tables.)