Stable environmental isotope techniques contribute to reasonable separations of the direct runoff component in snowmelt hydrographs. For the separation procedure, the actual isotope input from snow cover outflows is required. In order to study the effects of isotope fractionation and exchange processes in the snow cover on the isotopic outflow concentrations several cold room experiments have been carried out with isotopically homogeneous and stratified snow columns. To simulate natural conditions the columns were treated with different heat supply and rainwater at the surface, and the outflows analysed for 2H and 18O contents. Some fundamental results are discussed with respect to the more complex natural situation. Finally, the hydrological application of such experience is demonstrated for a natural environment.