Transient molecular flow studies of CH4, CD4 and mixtures using a Pyrex glass capillary were carried out under UHV-conditions at temperatures between 77.4 and 115 K and pressures between 10-5 and 10-3 Torr (10-3<0<10-1). The simultaneous recording of different components of the gas mixtures is achieved by means of a quadrupole mass spectrometer. The temperature dependence of the observed adsorption isotope effects is discussed in terms of the changes of the internal and external vibrations of the molecules caused by adsorption. The behaviour of gas mixtures was simulated by a numerical calculation.