Measurements of superimposed ice formation and snow properties as a function of the surface energy balance during melt-onset are presented. They were performed on fast ice on Kongsfjorden, Svalbard, between late May and early June 2002. On May 27, rapid snow melt commenced and within 5 days the snow cover initially 0.23 m thick transformed completely into 0.05–0.06 m of superimposed ice. The superimposed ice formed by both percolation of melt water to ice layers and by settling of snow between ice layers. Melt-onset was characterized by rapid changes in the total energy balance, which became positive throughout the whole day after May 27. The increased energy fluxes were mainly caused by higher incoming longwave radiation due to overcast conditions. The observations show that superimposed ice contributes significantly to sea ice mass balance.