Constant moisture content in a photographic gelatin can be established experimentally by sealing the gelatin in a vapour tight container with little free air volume compared to the volume of gelatin. Moisture content isolines can then be determined by varying the temperature and measuring the relative humidity in the free space after thermal and vapour phase equilibrium is reached. Surprisingly, the moisture content isolines reveal the glass transition temperature of the gelatin and its strong dependence on moisture content. The glass transition is correlated with the Flory-Huggins interaction parameter for a gelatin-water system, and the effects of water on the physical properties of the gelatin-water system are presented. Finally, the relationship between moisture content, relative humidity, and temperature is discussed with respect to accelerated ageing tests of photographic materials and long term storage in cold temperature environments.