The paper discusses the effects of a 4-year natural ageing on hydrothermal stability of new and previously accelerated aged parchments and leathers. It was observed that natural ageing generally lowers the denaturation temperature (measured using DSC), but in a different way for the two types of these collagen-based materials. Thus, for parchments the largest effect (up to 7 degrees C) was observed for the new samples, while for the strongly aged ones the temperature shift was negligible. For leathers, no relevant dependence of the denaturation temperature shift (3.0 +/- 1.0 degrees C in average) with the previous accelerated ageing time was observed. For parchment, the enthalpy of denaturation decreases substantially following natural ageing, while for leathers no significant alteration was observed. It was also found that the 1-day accelerated ageing performed in 2013 had comparable effects on the denaturation temperature of these materials as the 4-year natural ageing, making the possibility of predicting the effects of natural ageing on patrimonial artifacts manufactured from parchment and leather.