Abstract—The interpretation of the viscosity dependence of anthracene triplet decay is shown to be complexed by the effects of the macroscopic and the microscopic viscosity of the system on the translational diffusion coefficient. The second order rate constant for the decay of the anthracene triplet as measured by flash photolysis is equal to 2.45plusmn;.30×1091. mole‐1sec‐1for nearly a one hundredfold change in the viscosity of cyclohexane‐polyisobutylene solutions. The translational diffusion coefficient is unaffected by the presence of the polymer, and the local viscosity of the polymer solutions is identical to that of the pure