A new twin heat conduction microcalorimeter for the study of vapour sorption isotherms, sorption enthalpies and the kinetics of sorption has been developed. Each calorimeter has two calorimetric vessels which are connected by a steel tube. The upper vessel (a vaporization chamber) is charged with a vapour forming liquid and the lower vessel (a sorption chamber) contains the sorbent sample. An identical double calorimeter serves as a reference. During a measurement, the liquid in the upper vessel will vaporize and is allowed to diffuse to the sorption vessel. Thermal powers of vaporization and sorption are measured separately. The rate of vapour flow is obtained from a known value for the enthalpy of vaporization. Assuming equilibrium conditions and applying Fick’s law of diffusion makes it possible to calculate the sorption isotherm. Results from test measurements of water vapour sorption on samples of crystalline NaCl and KCl in saturated aqueous solution and on cotton wool are presented.