Solvent exchange on trans-[Os(en)(2)(eta(2)-H(2))S](2+) (S = H(2)O, CH(3)CN) has been studied in neat solvent as a function of temperature and pressure by (17)O NMR line-broadening and isotopic labeling experiments (S = H(2)O) and by (1)H NMR isotopic labeling experiments (S = CH(3)CN). Rate constants and activation parameters are as follows for S = H(2)O and CH(3)CN, respectively: k(ex)(298) = 1.59 +/- 0.04 and (2.74 +/- 0.03) x 10(-)(4) s(-)(1); DeltaH() = 72.4 +/- 0.5 and 98.0 +/- 1.4 kJ mol(-)(1); DeltaS() = +1.7 +/- 1.8 and +15.6 +/- 4.9 J mol(-)(1) K(-)(1); DeltaV() = -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.