One interestingway of measuring sorption isotherms and heats of sorption is the newly developed micro-calorimetric method. It turns out, however, that the adsorption kinetics must be proven to be rapid enough to fulfill the assumptions behind the theory for the determination of the isotherm to be valid. Here, the lower limit of the accepted rate of adsorption will be analyzed by making calculations of the diffusion behavior in the calorimetric vessel. The evaluation of the adsorption isotherms is based on assumptions of a quasi-static nature. If the adsorption kinetics are slow the behavior in the calorimeter will turn to a more or less transient process. When the heat of adsorption is of interest and the sorption isotherm and kinetics of sorption can be determined by other methods, compensation for the transient effects can be made.