Quantitative Coulometric analyses for water yield are made during thermal dehydration of rock materials and other solids. An electrolytic hygrometer cell is utilized to quantitatively measure the evolved water from a solid at known temperature as it is uniformly heated to a maximum of 1200°C. By reduction of total exposed adsorbing surface area in the retort and in the electrolytic cell coupling system, the lag between the dehydration event in the sample and cell response has been reduced to a very few seconds, thus permitting direct correlation between sample temperature and hygrometer cell current. The rapid cell response permits the resolution of discrete dehydration events and defines the temperature range through which they occur. Specific temperature‐desorption data gained with this instrument have been very helpful in gaining an understanding of the modes of constitution of water in silicates.