The excellent temporal resolution of event-related potentials (ERPs) makes it an ideal technique to test the timing of events within cognitive models which may not be distinguishable using behavioural data. Here a model of an ERP component is presented which tests features of the ACT-R cognitive architecture. The P300 is an event-related potential that is associated with attending to incoming stimuli and subsequent memory processing. It is commonly elicited using an oddball task in which a series of stimuli are presented comprising infrequent target stimuli against a background of frequent standard stimuli. It is influenced by the probability of a target in a sequence of stimuli. An ACT-R model was developed of the oddball task. P300 amplitude was correlated with the activation of the memory of the target stimuli, providing a good account of the component. The implications of these findings for the ACT-R architecture are discussed.