The authors show the implications that the free carrier trapping lifetime has on the capacitance-voltage (CV) characterization method applied to metal-oxide-semiconductor (MOS) structures. It is shown that, whereas the CV characterization method for deducing interface state densities works well for Si, the generally used frequency range of 100 Hz-1 MHz is much less adapted to GaAs MOS structures. Only interface trapping states in very small portions of the GaAs bandgap are measured with this frequency range, and mainly the very important midgap region is not properly probed. Performing an additional measurement at 150 degrees C on GaAs MOS structures eliminates this problem. (c) 2007 American Institute of Physics.