CW-ESR line broadening experiments are used to investigate the kinetics of fast hydrogen atom self-exchange reactions. The system NHPI/PINO˙ was studied in five different aprotic organic solvents at room temperature with a focus on the influence of the viscosity of the medium. Our findings support the theoretical descriptions derived from the statistical dynamics of the impact of the reorganization of the medium. In an Arrhenius type description the influence appears in the preexponential factor as a linear dependence on the dynamic viscosity.