Relaxer ferroelectric-based complex perovskite 0.91Pb(Zn1/3Nb2/3)O-3-0.09PbTiO(3) [PZNT91/9] and 0.68Pb(Mg1/3Nb2/3)O-3-0.32PbTiO(3) [PMNT68/32] single crystals were grown by a modified Bridgman process and by the flux technique. Their thermal properties were measured by simultaneous microdifferential thermal analysis (DTA) and thermogravimetric analysis (TGA), combined with phase analysis based on X-ray powder diffraction patterns of thermally treated samples. Upon heating at atmospheric pressure, the perovskite PZNT91/9 crystals undergo successively a decomposition at 1148 degrees C and a peritectic melting at 1226 degrees C with the formation of a pyrochlore phase. A liquidus point was found at 1268 degrees C, The PMNT 68/32 crystals show a sharp peritectic melting at 1270 degrees C and a liquidus point at 1288 degrees C, The results on the melting behavior and phase stability of the morphotropic PZNT and PMNT crystals provide valuable data for guiding the crystal growth process.