Redox reactions involving the [Cu(dmp)(2)](2+/+) couple (dmp = 2,9-dimethyl-1,10-phenanthroline) in acetonitrile were examined at elevated pressures up to 200 MPa. Activation volumes were determined as -8.8 and -6.3 cm(3) mol(-1) for the reduction cross-reaction by [Co(bipy)(3)](2+) (bipy = 2,2'-bipyridine) and for the oxidation cross-reaction by [Ni(tacn)(2)](3+) (tacn = 1,4,7-triazacyclononane), respectively. The activation volume for the hypothetical gated mode of the self-exchange reaction estimated from the reduction cross-reaction was -13.9 cm(3) mol(-1), indicating extensive electrostrictive rearrangement of solvent molecules around the Cu-II complex during the change in the coordination geometry before the electron-transfer step. On the other hand, the activation volume for the self-exchange reaction estimated from the oxidation cross-reaction was -2.7 +/- 1.5 cm(3) mol(-1). Although this value was within the range that can be interpreted by the concept of the ordinary concerted process, from theoretical considerations it was concluded that the reverse (oxidation) cross-reaction of the gated reduction reaction of the [Cu(dmp)(2)(CH3CN)](2+)/[Cu(dmp)(2)](+) couple proceeds through the product excited state while the direct self-exchange reaction between [Cu(dmp)(2)(CH3CN)](2+) and [Cu(dmp)(2)](+) proceeds through an ordinary concerted process.