During isotherm gas drawing of extremely dried gases (purity degree 6.0 or better) on pressure gas cylinders up to a pressure of approx. 10 bar the pressure-dewpoint in the cylinder remains constant. Because of desorption processes and the dependence of the enhancement factor on the pressure, neither the water vapour pressure in the cylinder, nor the moisture concentration in the drawn gas is constant. At a constant temperature in compressed gases not only the saturation water vapour pressure above water resp. ice, but also the equilibrium vapour pressure above an adsorbed water layer is a function of gas pressure, because of the influence of the enhancement factor. At isotherm pressure changes above unsaturated adsorbents not the water vapour pressure, but the pressure-dewpoint-temperature remains constant. Hygrometers based on adsorption principles (e.g. Al2O3-hygrometers) have a dependence on gas pressure, which is also explainable with the enhancement factor.