The domain structures, phase transitions and dielectric relaxation of relaxor-based piezo-/ferroelectric (1-x)Pb(In1/2Nb1/2)O-3-0.33Pb(Zn1/3Nb2/3)O-3-xPbTiO(3) (x=0.30, 0.34, 0.37, and 0.42) single crystals, abbreviated as PIN-PZN-PT, grown by flux method, have been analyzed by polarized light microscope and dielectric spectroscopy. The dielectric relaxation was described by the Curie-Weiss law and Lorentz-type quadratic equation. The substitution of Ti4+ ions for the B-site complex (Zn1/3Nb2/3)(4+) and (In1/2Nb1/2)(4+) ions results in a long-range symmetry breaking, as revealed by the formation of birefringent domains. Single crystal of PIN-PZN-PT with morphotropic compositions exhibit complex domain structures, which are composed of both rhombohedral and tetragonal phases intimately mixed together. The domain structures, the sequence and temperature of phase transitions have been interpreted in relation to the morphotropic phase boundary behavior of the PIN-PZN-PT system. The analysis of morphotropic domain structures and phase transitions provides a better understanding of the microscopic mechanisms of the enhanced piezoelectric properties recently disclosed in the PIN-PZN-PT and other PZN-based piezocrystals. (C) 2016 Elsevier B.V. All rights reserved.