[en] The isotope exchange reaction in equimolar H₂/D₂ mixtures has been studied in a static system over the temperature range 904 to 1220 K. The effects of excess argon and trace amounts of oxygen have been studied. The four centre molecular exchange mechanism is shown to be unobservably slow. The results are in quantitative agreement with the three centre atomic chain mechanism. Numerical integration of the differential rate laws for this mechanism has been used. The rate constants for the three centre exchange steps derived are in excellent agreement with theory and with direct measurements at low temperatures, and show that over the temperature range 274 to 1220 K there is upwards curvature in the Arrhenius plots corresponding to > T² dependence of the pre-exponential factors. For the reactions (3) D + H₂ → HD + H and (4) H + D₂ → HD + D; log₁₀(k₃/cm³ mol^-1s^-1) = (5.68 +- 0.44) + (2.51 +- 0.14)log₁₀(T/K) - (4950 +-140)/4.575 7(T/K); and log₁₀(k₄/cm³ mol^-1s^-1) = (3.36 +-0.53) + (3.21 +- 0.16)log₁₀(T/K) - (5664 +- 175)/4.575 7(T/K). (author)