The chemical environment of some Japanese vein-type deposits has been estimated on the basis of observed chemical composition of coexisting acanthite, sphalerite, and electrum and mineral stability relationships hitherto established. Selenium content of acanthite is relatively variable from 0.0 to 10 weight percent. The iron content of sphalerite coexisting with pyrite ranges from 0.0 to 8 weight percent. Silver content of electrum ranges from 53 to 73 weight percent. Theoretically the iron content of sphalerite and the silver content of electrum can be represented as a function of total dissolved sulfur content, pH, ionic strength, oxygen fugacity, and temperature. The selenium content of acanthite relates to these same variables and total dissolved selenium content. The common occurrence of alularia and sericite in these deposits gives the relation among pH, K (super +) concentration, ionic strength, and temperature. The chalcopyrite-pyrite assemblage restricts the probable f (sub O 2 ) -pH range at constant total dissolved sulfur content, ionic strength, and temperature. It is concluded that the Au-Ag vein-type deposits have been formed under relatively oxidizing conditions, whereas Pb-Zn vein-type deposits have been formed under relatively reducing conditions.