Interruptions are characterized by a brief suspension of a primary task due to the handling of a secondary task. Prior research indicated advantages of the possibility to prepare for an interrupting task. Here, we investigated the neurocognitive processes underlying such cognitive preparation by means of the EEG. We focused on the lateralization of alpha power relative to the position of task-relevant information as a correlate of the spatial focusing of attention. This oscillatory effect was measured during a working memory task based on lateralized visual stimuli which was interrupted by a 2AFC arithmetic task. Before every ten trials, an announcement was made to the participants whether they would be interrupted or not (always interrupted, never interrupted, randomly interrupted). First results pointed toward a decrease in performance in the primary task following interruptions, which was more pronounced when interruptions could not be anticipated. On EEG level, there was a stronger suppression of alpha power at posterior recording sites contralateral than ipsilateral to the side of task-relevant information in an initial memory array. This lateralized effect re-appeared following the completion of the interruption task, reflecting a re-focusing of attention on the primary working memory task. Importantly, contralateral alpha power suppression was stronger when participants could anticipate the interrupting task, indicating that cognitive preparation helps with the re-focusing of attention on task-relevant information after interruptions.