The diffusion of arsenic in polycrystalline silicon films is studied using a novel bilayer structure consisting of a polycrystalline silicon layer, in situ doped with arsenic, deposited on top of an undoped polycrystalline silicon layer. This technique avoids the complication of structural changes resulting from the ion implantation doping that has been employed in other investigations. The diffusivity is measured for the relatively low temperature range 700-850°C and is described by the relation D=10 exp (-3.36/kT)cm 2/s. The average deviation from this relation of the measurements from samples prepared in two different deposition systems is less than 20%. The values are about three orders of magnitude greater than the intrinsic diffusivity of arsenic in the silicon lattice. It is proposed that the diffusion takes place along grain boundaries and that certain background impurities in the grain boundaries are responsible for the large variation in the values that are reported in the literature.