This study reports the results of experiments that probed how solvents affect the recombination efficiency (F-cP) of geminate radical cage pairs. The macroviscosity of solvents has traditionally been used to make quantitative predictions about Fa, but experiments reported here show that Fa varies dramatically for solvent systems with identical macroviscosities. Experiments show that Per, correlates with the solvent microviscosity: five different solvent systems (consisting of a solvent and a structurally similar viscogen) were examined, and F-cP was the same for all five solvent systems at any particular microviscosity. The translational diffusion coefficient of the radicals (measured by DOSY) in the solvent system was used to define the microviscosity of the solvent system.