The degassing of magmatic volatiles during eruptions beneath ice sheets and glaciers, as recorded by the dissolved volatile content quenched in volcanic rocks, could provide powerful new constraints on former ice thicknesses in volcanic areas. As volcanic rocks are readily dateable using radiometric methods, subglacial volcanoes may therefore provide crucial information on the timing of palaeo-environmental fluctuations in the Quaternary. Volatile degassing is also likely to control the mechanisms of subglacial eruptions and their associated hazards. In this paper we lay out a number of criteria that must be satisfied for degassing to potentially record palaeo-ice thicknesses, using a variety of new datasets and calculations to highlight existing problems with the technique. These include uncertainties about volatile solubilities, non-equilibrium degassing, sample heterogeneity, hydration, post-quenching movement and whether subglacial pressures deviated significantly from glaciostatic. We propose new strategies for improvement of the technique and discuss how magmatic volatiles may control the style of subglacial eruptions.