The feedback-related negativity (FRN) has been hypothesized to be linked to reward-based learning. While many studies have shown that the FRN only occurs in response to unexpected negative outcomes, the relationship between the magnitude of negative prediction errors and FRN amplitude remains a matter of debate. The present study aimed to elucidate this relationship with a new behavioural procedure that allowed subjects to predict precise reward probabilities by learning an explicit rule. Insight into the rule did not only influence subjects' choice behaviour, but also outcome-related event-related potentials. After subjects had learned the rule, the FRN amplitude difference between non-reward and reward mirrored the magnitude of the negative prediction error, i.e. it was larger for less likely negative outcomes. Source analysis linked this effect to the anterior cingulate cortex. P300 amplitude was also modulated by outcome valence and expectancy. It was larger for positive and unexpected outcomes. It remains to be clarified, however, whether the P300 reflects a positive prediction error.