Event-related brain potential paradigms for the detection of concealed information commonly involve presenting probes embedded within a series of irrelevant items. This study investigated the impact of similarity of the irrelevant items with the probe. For the task, a card was shown followed by the sequential presentation of six "test" cards, one of which was the same as the initial card (the probe) along with five "irrelevant" cards that varied in terms of similarity with the probe. Participants either identified or denied recognition of the probe. The results show that P300 amplitude is modulated by stimulus similarity and highlight the importance of the irrelevant items on deception detection rates.