The decay time constants of the transient photo conductivity for six polycrystalline silicon wafers of different grain sizes are analyzed as a function of temperature (90 to 450 K). The photo conductivity decay is split into three components, namely, surface, shallow traps and deep traps in grain boundary, which are analyzed in terms of the effect of surface recombination velocity, changes in carrier mobility and grain boundaries. This is utilized to evaluate the temperature dependence of mobility and grain boundary potential in polysilicon samples. © Les Éditions de Physique 1996.