Solvent exchange on trans-[Os(en)(2)(eta(2)-H-2)S](2+) (S = H2O, CH3CN) has been studied in neat solvent as a function of temperature and pressure by O-17 NMR line-broadening and isotopic labeling experiments (S = H2O) and by H-1 NMR isotopic labeling experiments (S = CH3CN). Rate constants and activation parameters are as follows. for S = H2O and CH3CN, respectively : k(ex)(298) = 1.59 +/- 0.04 and (2.74 +/- 0.03) x 10(-4) s(-1); Delta H-double dagger = 72.4 +/- 0.5 and 98.0 +/- 1.4 kJ mol(-1); Delta S-double dagger = +1.7 +/- 1.8 and +15.6 +/- 4.9 J mol(-1) K-1 Delta V-double dagger = -1.5 +/- 1.0 and -0.5 +/- 1.0 cm(3) mol(-1). The present investigation of solvent exchange when compared with a previous study on substitution reactions on the same complexes leads to the conclusion that substitution reactions on these compounds undergo an interchange dissociative, I-d, or dissociative, D, reaction mechanism, where solvent dissociation is the rate-limiting step.