In this Letter we present, for the first time, VUV (109–220 nm) photoabsorption spectra of high density amorphous and crystalline forms of O3 relevant to both planetary/satellite and InterStellar Medium (ISM) conditions. Absorption in the Hartley region was observed to be very weak or absent when pure O3 ice was formed in the crystalline phase. Our results suggest that pure solid O3 effectively absorbs in the Hartley band but only at temperatures