The application of PDS (photothermal deflection spectroscopy) to the counting of defect states is complicated in a-GexSi1-x by the coexistence of two types of dangling bonds and their unknown energetic position in the forbidden energy gap. Using a model density of states, the authors conclude that it is intrinsically impossible to distinguish between Si and Ge dangling bonds. In addition, despite the fact that the density of bulk defects can be evaluated successfully, it is suggested that there is always a spurious absorption due to surface states. The measured density of dangling bonds keeps increasing from pure a-Si:H (Nd ≅ 2 × 1016 cm-3) to pure a-Ge:H (Nd ≅ 5 × 1017 cm-3), and this is shown to be a material property.