Activation energies, enthalpies, and entropies for the addition of phenylchlorocarbene to tetramethylethylene significantly decrease as the reaction solvent lengthens from n-pentane to n-octane to n-decane; additional decreases are minimal in n-pentadecane and n-heptadecane. Electronic structure calculations, employing a continuum solvent model, fail to reproduce the observations; instead, a qualitative model invoking solvent cage effects is proposed.