The fumarolic condensate and hydrogen gas samples collected from the Showashinzan, Nasudake, Yakedake and Kuju-Ioyama volcanoes, Japan, have been analyzed for the D/H ratio. A comparison between temperatures for isotopic equilibrium and measured outlet temperatures indicates that in high temperature fumarolic gases the deuterium exchange reaction between water vapor and hydrogen gas is rapid enough to readjust the equilibrium to the outlet temperature of fumaroles. For low temperature fumarolic gases, however, the isotopic temperature refers to equilibrium conditions deep in the fumarole.