The enantioselectivity of the cyclopropanation of styrene and methyl phenyldiazoacetate in the presence of a dirhodium carboxylate catalyst was found to be dependent on both the dielectric constant and the coordinating ability of the solvent. The enantiomeric excess is pressure dependent in supercritical fluoroform, changing from as low as 40% ee at pressures above 100 bar to nearly 80% ee at pressures near the critical point. The pressure dependence is caused by the changing dielectric constant of the fluoroform. Reactions in some coordinating liquid solvents had greater enantioselectivities than expected on the basis of dielectric constant alone. The crystal structure of the catalyst was obtained along with spectroscopic data in order to elucidate the causes of the solvent-dependent enantioselectivity.