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The enhanced grain boundary diffusion in cw laser processed LPCVD polycrystalline silicon films on insulating SiO2 has been investigated and the grain boundary diffusion coefficient D' for Phosphorus has been determined. Mesa diodes were fabricated in the large grain poly-Si films by standard photolithographic and etching methods and subsequent ion implantation. The electron beam induced current mode (EBIC) of the SEM allowed direct measurement of the dopant diffusion length along the grain boundaries intersecting the p-n junction. We have found a t¼ dependence of the penetration depth and Arrhenius behavior for the temperature dependence of the diffusion coefficient. The numerical evaluation of the experimental data is based on Whipple's exact solution of the grain boundary diffusion problem.