In this paper we present a summary of a study of reverse-type avalanche photodiodes (APDs) used as scintillation detectors for X- and γ-ray spectroscopy. The energy resolution of the APDs was evaluated by varying the avalanche gain, M, in the range from 1 to 200. The best energy resolution recorded for the 662 keV 137Cs γ-ray photoabsorption peak was 8.3 ± 0.5 % using a 5 × 5 × 5 mm3 BGO crystal coupled to a S8664-55 APD with a 5 × 5 mm2 light sensitive window, manufactured by Hamamatsu Photonics, Inc. The measurement was made with a gain of 20 and at a temperature of -20 °C. The main advantage of this method is that an accurate value for the intrinsic resolution of the crystal can be easily obtained by subtracting the contribution to the energy resolution from circuit noise and the electron-hole pair statistics. The intrinsic energy resolution of the BGO crystal sample at 662 keV was found to be 6.5 ± 0.5%. The observed intrinsic energy resolution of BGO as a function of energy shows the same relationship as that shown in other published works. With this method the intrinsic energy resolution of GSO (Ce 0.5 % mol) was also measured. While the intrinsic energy resolution of BGO shows an energy dependence proportional to E-1/2, GSO has a step-like characteristic, with the energy resolution increasing from ∼100 keV up to ∼500 keV.