We have used the methods of picosecond time‐resolved reflectivity to measure the carrier lifetime in fine grain polycrystalline silicon films grown by low pressure chemical vapor deposition at 625 °C. After monatomic hydrogen diffusion or implantation with phosphorus ions followed by high temperature annealing (1150 °C), the trapping time &tgr; increased from 40 to 150 ps, consistent with passivation of the grain boundaries or an increase in grain size, respectively. If implantation was not followed by annealing, &tgr; decreased to less than 10 ps, while if it was followed by low temperature annealing (900 °C), which approximately restored the original grain size, &tgr; recovered to 50 ps, very close to the trapping time measured in the as‐grown samples. In all cases, we found indications that trapping of carriers was much faster than their subsequent recombination.