A calorimetric solvent activity meter has been developed which allows fast and accurate measurements of the solvent activity as a function of solute concentration. By the use of the Gibbs-Duhem equation, the activity of the solute can be determined. The principle of operation is to measure the heat flow associated with the isothermal evaporation of solvent using a flow of dry nitrogen gas. The calorimeter consists of a precise thermostatic bath with a twin heat-conduction calorimetric unit. Special care has been taken to build the gas-handling system. The solvent activity is determined from the measured difference in thermal power for the evaporation from a solution and from the pure solvent. The solute concentration is changed by the stepwise addition of known amounts of concentrated solution to the calorimetric cell. The instrument has the potential of being both sensitive and precise and allows measurements to be made faster and more conveniently than by using the presently available methods.