Two-process theories of human cognition, that state that learning can occur by both associative and rule-based processes, are currently popular. We report two experiments which support such a view. Both employed a set of six stimuli which varied along a luminance dimension, and followed the same general design. That is, participants were trained to discriminate between the two stimuli in the middle of this set, before being tested on the whole set. In Experiment I, the length of training was varied. Following short training, participants' performance on test exhibited a peak-shift, and therefore may be explained in associative terms. After longer training, however, their behavior was consistent with rulebased learning. In Experiment II, the contingency during the training phase was varied. Participants in the 'Full Contingency' group performed in a manner consistent with rule-learning, while the 'Reduced Contingency' condition produced a peak-shift. These results are discussed in terms of McLaren, Green & Mackintosh's (1994) version of the associative/rule-based distinction.