This paper reports on the phase formation of perovskite Pb(In1/2Nb1/2)O-3-Pb(Zn1/3Nb2/3)O-3-PbTiO3 (PIN-PZN-PT) powder when doped with 0.04 to 0.83mol% ZnO. Air calcination of undoped powder mixtures for 4hours at 800 degrees C resulted in a mixture of Pb2Zn0.29Nb1.71O6.565 pyrochlore, PIN-PZN-PT perovskite, and In2O3. ZnO dopant concentrations as low as 0.04mol% increased the rate of perovskite formation and resulted in near phase pure perovskite powder of 0.5m particle size when heated at 800 degrees C in air. In all cases PbTiO3 and Pb(In1/2Nb1/2)O-3 formed prior to PIN-PZN-PT formation. ZnO doping promotes perovskite phase formation by increasing the reactivity of the intermediate pyrochlore phase by substituting Zn2+ on Nb5+ sites and forming oxygen vacancies when heated in air. Heating in high PO2 resulted in an incomplete reaction and a mixture of perovskite and pyrochlore whereas low PO2 resulted in phase separation into a mixture of rhombohedral perovskite, tetragonal perovskite, and pyrochlore. The PO2 sensitivity clearly shows that oxygen vacancies due to ZnO-doping are critical for synthesis of phase pure PIN-PZN-PT powder.