Far-infrared photothermal ionization spectroscopy has been used to investigate the impurities in selectively boron-doped GexSi(1–x)–Si strained-layer heterostructures. The spectra are obtained under various experimental conditions: with and without band-edge light and by varying the temperature of, and voltage applied to, the sample. The transport properties, resistivity, sheet charge density, and Hall hole mobility of these samples are presented. It is found that a sample with a two-dimensional hole gas (2-DHG) behavior at the GexSi(1–x)–Si interface has a weak photoresponse. This is due to a small number of photo-generated carriers in comparison with the residual hole carriers. For samples that do not show 2-DHG behavior, the photo-response from the substrate plays the dominant role for low applied voltages. At higher voltages, the response from the epitaxial layers becomes evident. The majority impurity is identified as the intentional dopant boron, and the dominant minority impurity is found to be phosphorus. It is concluded that although the free carriers are generated in the heavily B-doped layer, those carriers conducted through the low-energy high-mobility GexSi(1–x)–Si interface dominate the photoresponse.