Radionuclides released to the environment and deposited with or onto snow can be stored over long time periods if ambient temperature stays low, particularly in glaciated areas or high alpine sites. The radionuclides will be accumulated in the snowpack during the winter unless meltwater runoff at the snow base occurs. They will be released to surface waters within short time during snowmelt in spring. In two experiments under controlled melting conditions of snow in the laboratory, radionuclide migration and runoff during melt-freeze-cycles was examined. The distribution of Cs-134 and Sr-85 tracers in homogeneous snow columns and their fractionation and potential preferential elution in the first meltwater portions were determined. Transport was associated with the percolation of meltwater at ambient temperatures above 0 °C after the snowpack became ‘ripe’. Mean migration velocities in the pack were examined for both nuclides to about 0.5 cm h-1 after one diurnal melt-freeze-cycle at ambient temperatures of -2 °C to 4 °C. Meltwater fluxes were calculated with a median of 1.68 cm h-1. Highly contaminated portions of meltwater with concentration factors between 5-10 against initial bulk concentrations in the snowpack were released as ionic pulse with the first meltwater. Neither for caesium nor strontium preferential elution was observed. After recurrent simulated day-night-cycles (-2 °C to 4 °C), 80 % of both radionuclides were released with the first 20 % of snowmelt within 4 d. 50 % of Cs-134 and Sr-85 were already set free after 24 h. Snowmelt contained highest specific activities when the melt rate was lowest during the freeze-cycles due to concentration processes in remaining liquids, enhanced by the melt-freeze-cycling. This implies for natural snowpack after significant radionuclide releases, that long-time accumulation of radionuclides in the snow during frost periods, followed by an onset of steady meltwater runoff at low melt rates will cause the most pronounced removal of the contaminants from the snow cover. This scenario represents the worst case of impact on water quality and radiation exposure in aquatic environments.