Equipment is described for the study of the mois ture relations of materials in air of controlled relative humidities at elevated temperatures. The air is brought to the desired relative humidity and temperature by an arrangement including a saturator and automatically controlled heaters and then passed through a working chamber in which the materials to be studied can be weighed rapidly and accurately. Provision is made for measuring the rate of flow of air and for accurately de termining the relative humidity in the working chamber.The equipment has been used at temperatures from 96° F. to 300° F., relative humidities from 10% to 90% for temperatures up to 212° F. and to the maximum rela tive humidities obtainable at atmospheric pressure for temperatures between 212° F. and 300° F. The tem perature in the working chamber can be maintained con stant within ± 0.4° F. and the relative humidity within ± 1% relative humidity for experimental runs of several days' duration.Data are given illustrating the performance of the equipment.