The self-diffusion of 42K in potassium has been measured over the temperature range - 52.2 +/- + 61.8°C. The temperature dependence of the diffusion coefficient can be described by D=0. 16+/-0. 01 e-(9360+/-50)RT. However the deviations of the individual points from this Arrhenius line suggest that diffusion could be occurring by two mechanisms.