The balance described is intended for isothermal studies of moderately fast gas-solid interactions, such as adsorption, in which the heat of reaction is not negligible. The sample of solid is contained in a chamber which is integral with a hollow balance beam. The gas flow rate through the chamber is adjusted to minimize the resistance to heat and mass transfer in the boundary layer. The excess gas flow acts as a heat sink. The deflection of the beam under load changes is sensed by a frequency modulation capacitance system. Load changes of 15-250 mg give full scale deflection on a chart recorder and direct calibration and range reset by riders are available at any time. The balance has been used extensively in measurements of gas adsorption and diffusion in porous solids.