The thermal stability of the relaxor ferroelectric Pb(Zn1/3Nb2/3) O-3 (PZN) upon heating was studied using high-temperature X-ray diffraction (XRD) and scanning electron microscopy. It was found that single-phase PZN is stable below 700 degrees C, whereupon the first decomposition phase, the pyrochlore Pb-1.83 Nb1.71Zn0.29O6.39 was observed. With the increasing temperature, the amount of pyrochlore increased until at 1100 degrees C there is no PZN remaining. Pyrochlore formation was accompanied by the precipitation of ZnO above 800 degrees C and after holding at 1100 degrees C, the formation of Nb2ZnO6. Rietveld refinements based on the XRD patterns have allowed the relative phase proportions as well as the atomic site occupancies of the metal ions in PZN and the pyrochlore to be estimated. The Zn site occupancy within the perovskite PZN decreases upon heating in parallel with the pyrochlore formation. An elemental mass balance based on the XRD results shows that PZN decomposition is well under way before any Pb is lost from the sample. This indicates that Zn egress rather than Pb volatility is the determining factor in the instability of PZN, contrary to prior published work.