Thermal quenching of photoluminescence (PL) from SiGe/Si quantum well (QW) structures grown by molecular beam epitaxy is shown to be more severe when grown at a lower temperature. The mechanism responsible for the thermal quenching of PL is discussed as being due to thermally activated nonradiative recombination channels, related to defects in both Si barriers and SiGe QW. Nonradiative defects in Si can be rather efficiently deactivated by post-growth treatments such as hydrogenation and thermal annealing, leading to a significant improvement in the thermal quenching behavior of PL from single QW structures. Nonradiative defects in SiGe are found to be thermally stable, on the other hand, evident from the experimentally observed minor role played by post-growth thermal annealing in the thermal quenching of PL from multiple QW structures.