This study measured and compared the combination efficiencies (FcP) of geminate radical cage pairs to non-geminate (collisional) radical cage pairs (F-c'). For the [Cp'(CO)(3)Mo center dot, center dot Mo(CO)(3)Cp'] radical cage pair, Fc' was found to be smaller than FcP in solutions having the same viscosity. It is proposed that the difference in FcP and Fc' arises because the radicals in the collisional cage pair are less likely to have the correct orbital orientation for radical-radical combination to occur, whereas photochemically generated geminate cage pairs are more likely to have the correct orbital orientation. As predicted, F-cP and F-c' both increase when the microviscosity of a solution increases.