The electromagnetic field at frequency 2.45 GHz tends to orientate the dipolar molecules of dielectric materials; subsequent relaxation provokes very rapid volumetric heating. Due to the inverse thermal gradient, special circumstances occur during evaporation of saturated solutions: crystallization conditions are also modified. Nucleus formation in vitamin C and KCl solutions is studied under coupled convective and microwave energy input. The limit in microwave power density for vitamin C is the heat sensitivity of the material. In order to avoid irreversible changes intermittent microwave power has to be applied. Crystallization process is traced by measuring the temperature of the solutions and recording the absorbed microwave power. Quality of crystallization is assessed by scanning-electron microscopy and granulometry.