The damage induced by hydrogen and helium plasmas at the surface of crystalline silicon has been monitoredinsituby time‐resolved microwave conductivity and by spectroscopic ellipsometry measurements. Both plasma treatments increase the decay rate of the optically generated excess charge carriers and decrease the amplitude of the microwave reflection transients. While for the helium plasma a high density of electronic defects is created immediately after plasma ignition, a continuously increasing number of recombination centers is observed in the case of the hydrogen plasma exposure. In support of the transient microwave measurements, the analysis of the spectroscopic ellipsometry measurements reveals the creation of a damaged surface layer, which in the case of the helium plasma exposure has a high and in the case of the hydrogen plasma a low fraction of amorphous silicon. This can be explained by the different nature of the processes involved in the interaction of hydrogen (chemical) and helium (physical) plasmas with the silicon surface. After a constant plasma exposure time the damaged surface layer is thicker in the case of the hydrogen plasma exposure. Moreover, the helium plasma treatment produces a more defective overlayer as deduced from the faster decay of the transient microwave signals. ©1995 American Institute of Physics.