The reaction between OH radicals and [CUII(P2O7)(2)(H2O)(2)](6-) results in the formation of a Cu(III) complex. No reaction is observed with N-3(.) or Br-2(.-), whereas SO4.- initiates the same steps as seen with (OH)-O-.. This suggests that the mechanism probably involves a ligand interchange or H atom abstraction process. The Cu(III) complex undergoes a rapid first-order reaction, probably loss of a P2O74- chelate, followed by addition of OH- (pK(OH) approximate to 10) to yield a Cu(III) complex that is predominantly hydroxylated and has a relatively long half life. The effect of pressure (up to 150 MPa) on the formation and subsequent reactions of the Cu(III) complex is used to help elucidate the mechanism of the different steps.