Stopped-flow spectrophotometric measurements identify and determine equilibrium data for thiourea (tu) complexes of copper(II) formed in aqueous solution. In excess Cu(II), the complex ion [Cu(tu)](2+) has a stability constant beta(1) = 2.3 +/- 0.1 M(-1) and molar absorptivity at 340 nm of epsilon(1) = (4.0 +/- 0.2) x 10(3) M(-1) cm(-1) at 25.0 degrees C, 2.48 mM HClO4, and mu = 464 mM (NaClO4). The fast reduction of Cu(II) by excess tu obeys the rate law -d[Cu(II)]/dt = k’[Cu(II)](2)[tu](7) with a value for the ninth-order rate constant k’ = (1.60 +/- 0.18) x 10(14) M(-8) s(-1), which derives from a rate-determining step involving the bimolecular decomposition of two complexed Cu(II) species. Copper(II) catalyzes the reduction of hexachloroiridate(IV) by tu according to the rate law -d[IrCl62-]/dt = (k(2,unc)[tu](2) + k(1,cat) [tu](5)[Cu(II)])[IrCl62-]. Least-squares analysis yields values of k(2,unc) and k(1,cat) equaling 385 +/- 4 M(-2) s(-1) and (3.7 +/- 0.1) x 10(13) M(-6) s(-1), respectively, at mu = 115 mM (NaClO4). The corresponding mechanism has a rate-determining step that involves the oxidation of [Cu-II(tu)(5)](2+) by [IrCl6](2-) rather than the bimolecular reaction of two cupric-tu complexes.