Time-resolved infrared absorption has been applied to obtain elementary rates of unimolecular decomposition in the gas phase for the chromium carbonyl complexes of ethylene, propylene, l-butene, cis-2-butene, trans-2-butene, and isobutene. Observed rates fit a trend of declining stability with increasing alkyl substitution. Arrhenius parameters, derived from the temperature dependence of the elementary unimolecular decay rate, establish that the source of this stability trend lies not in decreasing bond strengths-activation energies are essentially constant for the series-but rather in a substantial increase in the A factor for the larger leaving olefins. This effect is explained in terms of sterically constrained torsional and C-C-C skeletal bending vibrations that are released as the molecule dissociates, adding to the statistical driving force that favors decomposition. This suggestion is confirmed by a simplified RRKM unimolecular rate theory model that quantitatively reproduces the observed rates.