The moisture-content isolines of a photographic gelatin were measured. Constant moisture content was established experimentally by sealing gelatin in a vapor-tight container with litter free air volume compared with the colume of the gelatin. The temperature was varied, and the relative humidity in the free space was measured after thermal and vapor phase equilibrium hadd been reached. A significant shift in the moisture vapor phase equilibrium over gelatin was found. Surprisingly, moisture-content isolines easily reveal the glass transition temperature of the gelatin, and the glass transition's strong dependence on moisture content was confirmed. Traditional moisture-absorption isotherms do not clearly indicate the glass transition temperature. The implications of these results are discussed with regard to interpretation of accelerated aging tests, to storage in unregulated environments, and to low-temperature storage of photographic materials.