Collagen and tanned collagens have been adjusted to various pH values and stored at 40°C. at different relative humidities. Hydrolytic breakdown of the protein as indicated by release of N-terminal residues increases with decrease in pH and with increase in relative humidity. When breakdown becomes extensive, denaturation follows and this in its turn accelerates further hydrolytic action. Cross-linking of the protein by a tanning agent reards hydrolytic breakdown during storage and also helps to maintain structure even when such breakdown is quite extensive. Oxidative breakdown is also thought to occur but no direct evidence of the presence of keto acids, one of the expected by-products of such degradation, was obtained. The implications of these findings to the deterioration of leather on exposure to warm moist conditions or acid atmospheres, and to the storage of -- AATA