Illegitimate tasks are tasks that violate norms for what the employee should do as part of the job, and have been found to harm employees' well-being. The current research uses a mixed methods design to examine the role of attributions on the two types of illegitimate tasks: unreasonable and unnecessary tasks. A sample of 432 engineers described a specific illegitimate task that was assigned to them, the attributions they made and their response. They also completed a quantitative questionnaire. Results from both the qualitative (event level) and quantitative (person level) portions of our study portray differences in the attributions made to unreasonable and unnecessary tasks, as well as differential negative effects on employees' emotions. In addition, hostile attribution bias was found to moderate the relationship between illegitimate tasks and negative emotions, particularly for unreasonable tasks. This supports the theoretical basis for illegitimate tasks because unreasonable tasks pose a potentially greater risk to the employee's self-worth than unnecessary tasks that are more often assigned at random.