Two important assumptions of behavioral momentum theory are contradicted by existing data. Resistance to change is not due simply to the Pavlovian contingency between a discriminative stimulus and the rate of reinforcement in its presence, because variations in the response-reinforcer contingency, independent of the stimulus-reinforcer contingency, produce differential resistance to change. Resistance to change is also not clearly related to measures of preference, in that several experiments show the two measures to dissociate.