The general scaling laws for transient one- and two-phase mass and heat flow, and the other conditions that hold in the transitional state, have been published. Details of the scaling of loss-of-coolant accident phenomena for water-cooled nuclear reactors are investigated by means of experimental loops using water and Freon-12. To a smaller extent we have also investigated the scaling of heat- and mass-flow transients in sodium-cooled nuclear reactors by using water.