A high-pressure, high-temperature investigation of a glass prepared from a natural garnet-bearing rhyodacite from Victoria has been conducted under anhydrous and hydrous conditions (2, 5 and 10% by weight of water added). Garnet and quartz are near-liquidus phases from 13.5 to 27 kb with 2% water present, but with 5–10% water present garnet is the liquidus phase from 18–27 kb and the appearance of quartz is depressed significantly below the liquidus. All garnet crystals analysed over the pressure range 11 to 27 kb have > 12 mol.% grossular, higher than that found in the natural garnets. It was not possible to obtain garnets with composition corresponding in grossular content and Fe/Mg ratio to the natural garnets for the experimental conditions of this study.