The importance of the moisture and oil fat content of leather is described. Leather is composed of tanned collagen, moisture, oils and fats. These components should be in equilibrium between ph 3-6, with water comprising 12-20% and fat 2-10%, depending on the type of leather. Within these limits the leather is stabilised, outside them destabilised. When the fat content is too high, there is reduced capacity to hold water, leading to cross-linking directly between fibres, with loss of flexibility and hardening. The method employed for determining moisture content is based on the alca manual, but uses dry heating at 95 c for 4 hours to avoid loss of volatile oils. Moisture retention is measured by equilibrating samples at different rhs and determining weight changes. The importance of water in leather structure is emphasized by comparison with other natural materials and figures given at 50% rh for normal moisture content. Graph.