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This paper reports on the pressure dependence of the optical absorption edge of ZnO in the wurtzite and rock-salt phase, up to 14GPa. Both vapor-phase monocrystals and pulsed-laser-deposition thin films have been investigated. In both types of samples the wurtzite to rock-salt transition is observed at 9.7±0.2GPa. The absorption tail of the fundamental gap, as measured in monocrystals, exhibits a pressure coefficient of 24.5±2meV/GPa. The evolution under pressure of the full absorption edge of the wurtzite phase is studied with thin film samples, yielding a slightly lower pressure coefficient (23.0±0.5meV/GPa for the A–B exciton). Rock-salt ZnO is shown to be an indirect semiconductor with a bandgap of 2.7±0.2eV. At higher photon energy a direct transition (E_gd–4.5eV) can be also identified in thin films transited to the rock-salt phase. Results on the high-pressure phase are interpreted on the basis of density …