The effect of a large increase in mass and extension of substituents on the thermal rearrangement of a 1,2-divinylcyclobutane system has been investigated by the introduction of two 3-cholest-4-ene units. The ratio of two types of exit channel, fragmentation and stereomutation (Fr/St), from a widely accepted diradical intermediary (caldera) has increased significantly relative to the ratio in a simpler system. We believe this to be the first example of the influence of a ponderal effect on the ratio of two competing processes in a thermal rearrangement. The internal torsional rotations necessary prior to reclosure for the realization of stereomutation have been markedly depressed. One of the three stereoisomeric cyclobutanes reacts substantially more slowly than the other two. In its structure, determined by X-ray diffraction, the two cholestenyl wings are folded closely together within van der Waals radii. The slower reaction may reasonably be ascribed to a relative lowering of heat of formation in this ground-state conformation.