The cis−trans chloride isomerization of a ruthenium olefin metathesis catalyst is studied using quantum mechanics (B3LYP DFT), including the Poisson−Boltzmann (PBF) continuum approximation. The predicted geometries agree with experiment. The energies in methylene chloride, lead to ΔG = −0.70 kcal/mol and a cis:trans ratio of 76:24, quite close to the experimental value of ΔG = −0.78 kcal/mol or c:t 78:22. In contrast, we predict that in benzene c:t = 4:96 in agreement with the experimental observation of only the trans isomer. Our calculated relative activation energies explain the observed difference in initiation rates and suggest that each isomer should be isolable in high ratio by simply changing solvent.