Exchange of deuterium between deuterium oxide and molecular hydrogen is induced by gamma rays from Co60. The reaction was studied by irradiating heavy water solutions saturated with hydrogen gas, and then analyzing the gas with a mass spectrometer. The reaction rate conforms to the exponential exchange law. The exchange yield is independent of dose rate and of hydrogen concentration over a moderate range. Prolonged irradiation leads to a steady state where the isotopic composition of the gas is close to that of the solution. This state differs greatly from that for thermodynamic equilibrium at room temperature.