Combining a classical test theory and an item response theory (IRT), this study aimed to investigate the psychometric properties of the Bern Illegitimate Tasks Scale (BITS) by measuring two conceptually separate dimensions capturing unnecessary tasks (perceived by employees as pointless) and unreasonable tasks (perceived as unfairly or inappropriately assigned). Data collected among Polish employees in two samples (N= 965 and N= 803) were analysed. Results from the classical test theory (parallel analysis, exploratory and confrmatory factor analyses) indicated two correlated factors with four items each, confrming the theory of illegitimate tasks. This study is the frst to report item and scale functioning using IRT analysis of each of the two dimensions of BITS. All items on each dimension had acceptable discrimination and difculty parameters. Moreover, items had measurement invariance between men and women. All levels of unnecessary and unreasonable tasks were reliably captured by BITS items. Convergent and discriminant validities of both dimensions of BITS were confrmed in relation to work overload, work performance and occupational wellbeing. We conclude that BITS, in the case of the Polish version, is psychometrically suitable to use with the working population.