The ac impedance has been considered in a general way which includes the effect of a distribution of trapping states within Schottky-type energy barriers. Starting with the complete unipolar transport equation, and introducing a density-of-trapping-states function, depending generally on both position and energy, one arrives at a useful formulation of the ac impedance. This result is possible because of the rapid variation of the effect of traps in such barriers which restricts their main contribution to a narrow range of positions and energies. Thus, expressions are obtained which relate the ac impedance measurements to the trapping effect at regions of the barrier corresponding to the applied frequency, voltage, and temperature. With these three independent variables, it is possible to experimentally scan the barrier over energy and position, obtaining a profile of trap densities and other barrier properties as well as independent tests on the selfconsistency of the method. Some experimental results obtained from thin-film Nb2O5 capacitors are given and shown to be in general agreement with the theory. This agreement also supports our contention, based on previous dc studies, that the Nb2O5 film contains a large ionic space charge. From the results we calculate a barrier minimum of 0.38 eV, an average trap density of about 1019 cm−3 eV−1, and trap capture cross section of about 10−13 cm2.