The thermal stability of crystallite powder and bulk single crystals of relaxor 0.91Pb(Zn1/3Nb2/3)O-3-0.09PbTiO(3) (0.91PZN-0.09PT) solid solution in air and an PbO-rich environment has been investigated. At 700degreesC, perovskite PZN-PT decomposes only slightly to the pyrochlore phase. At 800degreesC, the rate of decomposition is accelerated, promoted by a surface breakaway decomposition process. This process occurs via the inward growth of faceted pyrochlore grains from the particle surface. At a certain point of the growth process, they will break away and detach themselves from the perovskite phase. The transformation stress also causes the adjacent perovskite phase to fracture concurrently, contributing to the breakaway event. At higher annealing temperatures (i.e., 900degrees-1000degreesC) and/or in the PbO-rich environment, a layer of PbO-rich liquid phase is formed on the surface of the particle. Because of limited wetting between the PbO-rich liquid phase and the earlier-formed pyrochlore grains, the latter detach themselves from the perovskite substrate. This action frees the substrate from the pyrochlore nuclei to initiate the breakaway decomposition event, which causes the rate of decomposition to slow substantially. At 1100degreesC both in air and the PbO-rich environment, the PbO-rich liquid formed vaporizes readily, which causes the decomposition rate to accelerate again. The present work shows that (i) the decomposition of perovskite PZN-PT to the pyrochlore phase during high-temperature annealing is a surface phenomenon and (ii) its rate is controlled by the relative rate of formation and vaporization of the protective liquid layer that is present under the annealing conditions.