Surface passivation in silicon wafers is investigated by contactless photoconductance measurements. It is shown that the deposition of a-Si:H layers on top of c-Si wafers leads to passivation of the c-Si surface. Passivation by Si3N4 of the p c-Si surface is at least partially due to the depletion layer at the interface. The separation of excess charge carriers by the depletion field is responsible for a relatively slowly decaying part of the signal not directly related to bulk recombination. An HF dip is not sufficient for an appropriate passivation of p c-Si but immersion in I2/ethanol solution is promising.