The capabilities of admittance spectroscopy for the investigation of a-Si : H/c-Si heterojunctions are presented. The simulation and experimental results, which compare very well, show that the admittance technique is sensitive to parameters of both the a-Si : H layer and the a-Si : H/c-Si interface quality. In particular, the capacitance versus temperature curves have two steps, accompanied by two bumps in the temperature dependence of the conductance. The first step occurring in the low temperature range (100-200 K) is related to the transport and response of gap states in the a-Si : H layer. The second step occurring at higher temperature (>200 K) is caused by a carrier exchange with interface states and appears when the interface defect density exceeds 5x1012 cm-2. Then, the interface defects affect the band bending, and thus the activation energy of de-trapping, which favours the exchange with electrons from the a-Si : H and holes from the c-Si, respectively, for increasing defect density.