The exchange coupling J(HH) in OsH(3)X(PH3)(2) (X = Cl and I) has been calculated by determining the eigenstates resulting from the coupling between the internal rotation and vibration modes, which correspond to the painwise hydrogen exchange, The potential has been calculated by core potential ab initio methods at the RHF/MP2, level. The height of the barrier for exchange of two adjacent hydrogen atoms has been calculated to be 12.9 kcal.mol(-1) (X = Cl) and 14.6 kcal.mol(-1) (X = I). The ratio between the calculated exchange coupling constants for X = Cl and X = I is found to be in good agreement with the experimental ratio though shifted up in temperature. The temperature dependence of J(HH) is calculated to be exponential, which is the typical behavior for such a coupling constant.