On the basis of the thermodynamic stability of hydrates of SO2, H2S and CO2, it is suggested that those compounds may be found in some terrestrial volcanic environments. Hydrates of SO2 and H2S may be found in low-temperature fumaroles, especially where surrounding rocks are below 0 ¿C. Such situations could occur at high altitudes and/or high latitudes. Hydrates of H2S, SO2, and CO2 may be found on grain surfaces and in pore spaces of pyroclastic or other material overlying gas sources where proper conditions prevail, such as pore pressure equal to overburden pressure. For H2S, such deposits can occur with mean surface temperatures as high as 20 ¿C. Submarine volcanism could give rise to stable submerged hydrate occurrences analogous to both the above occurrences.