The chemical compositions of volcanic gases sampled in Japan are dominated by change in either the carbon dioxide and water or sulphur gas concentration with time. The gas compositions reflect the source magma compositions, which range from mafic (Tokachi) to felsic (Showahinzan), and processes (such as degassing and intrusion) that have occurred in magma chambers. Carbon dioxide originates from the mantle and marine carbonate in crustal rocks. After the historic eruptions of volcanoes here, magmatic vapour became progressively depleted in helium (R/Ra = 8 to 2), carbon (∞¹³C-5 to -11‰), hydrogen (∞D = -20 to -60‰)and oxygen isotopes (818O = +8 to -3‰) due to crustal and air saturated ground water contamination (He), degassing (C and H) and mixing with local meteoric and geothermal water (H and O). The aqueous alkali chloride species, CO₂ and S gases that exsolve from magmas beneath volcanoes disproportionate and dissociate in surrounding ground waters, and mix with solutes derived from marine formations where present.