Irritable mood is implicated in a range of psychiatric conditions in both adults and adolescents. Research into appropriate measures of irritability, however, has been sparse. Recently, Stringaris et al. (2012) published the Affective Reactivity Index (ARI), a measure of chronic irritability with promising psychometric properties. This article presents psychometric properties of the ARI with Australian adolescents and, for the first time, with adults. The adolescent sample (n = 396) was recruited from 11 secondary schools in southeastern Australia. The adult sample (n = 221) was recruited through poster and online advertising. Both samples completed a battery of measures (including the ARI, Reynolds Adolescent Depression Scale-2, Strengths and Difficulties Questionnaire, Center for Epidemiologic Studies Depression Scale, Generalized Anxiety Disorder Screen, and Liebowitz Social Anxiety Scale) on a single occasion, and a subsample of adults (n = 32) completed the ARI a 2nd time after 1 week to establish test-retest reliability. Parent and self-report scales had excellent internal consistency and correlated well with each other. Test-retest reliability was also very good in the adult sample (intraclass correlation coefficient = .80). Convergent validity was demonstrated, as irritability was related to psychopathology in both adults and adolescents as expected. The ARI is a brief, easy-to-use scale to measure chronic irritability, with promising psychometric properties for use with Australian adults and adolescents.