Diffusion of radioactive phosphorus in polycrystalline silicon has been studied in the temperature range 566-980 degrees C. Diffusion profiles were obtained by anodic oxidation sectioning coupled with radio tracer detection. Diffusion coefficients in the grain boundary were obtained following Leclaire's method of analysis. An Arrhenius plot of grain boundary diffusion coefficients gave rise to an activation energy of 2.65 eV and pre-exponential factor D' 0=4.8*10-3 cm2 s-1.