The infinite dilution activity coefficients for various types of systems, one-component solute + one-component solvent, one-component solute + multicomponent solvent, and multicomponent solute + solvent have been studied by use of the gas stripping method. Two techniques, the single-cell technique (SCT) and the double-cell technique (DCT), are described. The use of them depends mainly on the volatility and nature (one-component or multicomponent) of the solvent.