Abstract-The sulfur isotopic values of barite, gypsum and anhydrite from the typical strata-bound submarine ore deposits (black ore deposits) of Miocene age in Japan have a narrow range of +21 to 24%o with respect to the meteoritic sulfur. However, the oxygen isotopic values (in SMOW) vary from +5 to +19%o and have a general tendency to increase in the order : barite < anhydrite -alabaster < secondary gypsum (satin spar and selenite). The sulfate minerals from the late Paleozoic cupriferrous iron sulfide ore deposits of the Yanahara and Besshi mines are similar in the sulfur isotopic values (+11 to +15%o) to inferred Permian sea water sulfate, whereas the oxygen isotopic values have a wide range of variation.The observed sulfur isotopic pattern is most likely due to a submarine hydrothermal system in which sulfate of marine origin (connate or actual sea water) predominated over other sources of sulfur. The variable oxygen isotopic data of sulfate are explained by the oxygen isotopic exchange between sulfate and water during depositional and post-depo sitional processes. The sulfur and oxygen isotopic ratios of "hot sulfate" in the Wairakei and the Red Sea geothermal brines are consistent with this idea.The sulfates in secor dary gypsum ores in the Noto mine are enriched in both 34Sand 180, suggesting that the alteration of the primary ores in this mine may have taken place under the influence of sulfate-reducing bacteria.