Research has shown that different types of interruptions can affect their disruptiveness. However, it is unclear how different features of the interrupting task determine its disruptive effects. Specifically, some theories predict that the difficulty of an interruption does not contribute to the disruptive effects of that interruption alone. Disruptive effects can be mediated by the extent to which the interrupting task interferes with the ability to rehearse during the interruption. In this experiment participants performed a single primary task with three interruptions of different difficulty. We found that interruptions were more disruptive when the task minimized the participant's ability to rehearse (as measured by the number of mental operators required to perform the task) and not just when they were more difficult. These results suggest that the ability to rehearse during an interruption is critical in facilitating resumption of a primary task.