Lead zinc niobate-lead titanate (PZN-PT) single crystals are able to sustain very large piezoelectric strains, a phenomenon for which the term 'giant piezoelectric effect' was recently coined. PZN-4.5%PT crystals were studied by means of single-crystal neutron diffraction at temperatures between 293 K and 438 K. Two phase transitions were observed. The first, from the room temperature rhombohedral perovskite structure (R3m) to the tetragonal lead titanate phase (P4mm) occurs at 391 ± 2 K. The second, to the cubic perovskite aristotype, occurs at 421 ± 2 K. These temperatures are at slight variance with the current phase diagram. The transitions are relatively abrupt, occurring over a range of only 2-3 K. A monoclinic phase recently reported to occur between the rhombohedral and tetragonal phases in the lead zirconate titanate system (PZT) does not occur here. Group theoretical analysis using the computer program ISOTROPY indicates that the intermediate monoclinic phase cannot facilitate a truly continuous transition from rhombohedral to tetragonal symmetry.