The reaction of Cl atoms with intrinsic andn+‐doped polycrystalline silicon has been studied at a Cl partial pressure of 0.17 Torr and in the temperature ranges from 150 to 290 °C and 25 to 90 °C for the two materials, respectively. The reaction withn+‐doped silicon was observed to proceed 90 times faster than with intrinsic silicon at any given temperature, i.e. within experimental error the difference in the rate constants for the two materials was found to be entirely attributable to a change in the pre‐exponential factor, with the activation energy remaining unchanged. The rate constant for the reaction is given by (9±2)×105nm min−1 Torr−1 exp−28.2±1.2 kJ/mol)/RTfor the intrinsic material and (7±3)×107nm min−1 Torr−1 exp−(27.8±1.5kJ/mol)/RTfor the phosphorus doped material with a dopant density of 5×1018cm−3.