The thermally induced self-segregation (TISS) of the chemical composition in sodium niobate single crystals has been investigated by Auger spectrometry and secondary-electron emissiometry. The high efficiency of these methods for studying fine features of TISS processes has been confirmed. Comparative analysis of TISS in sodium and potassium niobates, having perovskite structure, is performed within the liquid-phase TISS model. The difference for these compounds is that excess sodium is accumulated on the surface of sodium niobate, whereas excess niobium is accumulated on the surface of potassium niobate. This distinction is explained by the higher potassium vapor pressure in comparison with sodium.