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The phase transition of zinc oxide from B4 (hexagonal wurtzite structure) to B1 (cubic rocksalt structure) has been studied by energy dispersive powder diffraction technique up to 11 GPa and 1273 K. Analysis of equation-of-state (PVT) data for the rocksalt phase yields precise values of the bulk modulus and its temperature derivative. The previously accepted P-T phase diagram is shown to be incorrect. It is established that the B1 phase is not recoverable. The equilibrium transition pressure of the B4-to-B1 transformation is near 6 GPa (at ambient temperature) and the dP/dT slope close to zero. These new results are confirmed by using simultaneously three other different types of experiment (imaging, ultrasonic and X-ray diffraction studies on single crystal specimens).