Two experiments are reported that investigate whether the logical equivalence of conditionals and disjunctives is paralleled by a psychological equivalence. In these experiments, subjects rephrased from one form into the other. Experiment 1 demonstrated strong effects of familiarity and causality of rule content. Similar ® ndings were found in Experiment 2 with a different conditional rule syntax. An account of the experiments is given in terms of mental models theory: In this account, task performance can be seen to depend upon the extent to which the model sets used by subjects to generate rephrasings are complete, task content being the most important factor affecting model set completion. A ``Minimal Completion’’ strategy is proposed to operate in the absence of thematic content. The experiments also falsify the long-held assumption that conditionals with negative antecedents are always interpreted as their disjunctive equivalents. This raises doubts about the mental models explanation for matching bias in conditional reasoning.