Uniaxial stress-strain measurements have been performed on two glassy polycarbonates and one polyimide in an atmosphere of CO2, H2O-vapour or acetone-vapour. The yield stress has been determined as a function of the concentration of sorbent molecules. In all observed cases the polymer is softened due to the sorption. The depression of the yield stress at a given concentration is dependent an the size of the sorbent molecules and of interchain volume sites within the polymer. For H2O and CO2 a unique relation is found between the softening and the volume dilatation accompanying the sorption. This dilatation has been associated with an internal isotropic stress via Hooke＇s law. That enables us to explain the softening as a consequence of the well known dependence of the yield stress on the isotropic stress component.