Medial-frontal negative potentials have been elicited by performance feedback in choice reaction time tasks and by monetary losses in simple gambling tasks. One theory of the functional significance of these potentials suggests it is responsive to the emotional significance of an event, whereas the Holroyd and Coles (2002) model posits that it is an error signal generated in response to detecting an outcome that is worse than expected. To investigate these theories this experiment involved participants learning probabilistic relationships between stimuli and monetary values. First, the participant was shown a shape; next, they indicated which monetary outcome they thought was associated with that shape and rated their confidence in their choice; then they were given accuracy feedback; and finally, they were presented with the associated monetary outcome. A major finding was that the amplitude of the ERN was greatest in response to error feedback stimuli at the beginning of learning the association, consistent with the Holroyd and Coles model. A further result, however, showed that the waveforms in response to monetary outcomes that were better than expected did not differ from those elicited by outcomes that were worse than expected.