Summary of semi-quantitative whole-rock XRD analysis of hydrothermal alteration across the island of Milos, Greece. The samples were acquired during numerous field visits between 2010 and 2018 by the British Geological Survey and GW4+ Doctorial Training Partnership (NE/L002434/1). The data were acquired using a PANAlytical X’Pert Pro diffractometer at the British Geological Survey, Keyworth, UK. These data were primarily used to identify mineral phases to improve our understanding of regional and local paleo-hydrothermal activity. This may be useful within the metallic and industrial mineral mining sector and associated researchers. The data includes grid references (± 5 m), field observations and instrument running conditions. Samples from 2010 are supported by a grant award from the Natural Environment Research Council (GA/09F/139). Samples from 2016 - 2018 are supported by the GW4+ Doctoral Training Partnership (NE/L002434/1) grant award GA/09F/139 – RMS E3557, and the British Geological Survey’s University Funding Initiative (BUFI S345).