The rate of the ligand substitution reaction of copper (II)-PAN (CuR) with EDTA (Y) has been determined spectrophotometrically in 5% v/v dioxan over the pH range 5.0-6.3 at mu = 0.1 (NaClO(4)) and at 25 degrees . In the absence of 1,10-phenanthroline the rate law is expressed as -d[CuR(+)]/dt = 10(3.2) [CuR(+) [Y'], and the release of PAN from the reaction intermediate CuRY is the rate-determining step. In the presence of 1,10-phenanthroline (X), however, copper forms a stable mixed-ligand complex (CuRX(+)), and the rate of substitution with EDTA is expressed as -d[CuRX(+)]/dt = (10(6.2)[H(+)] + 10(4.8)[X]) [CuRX(+). The release of PAN from the mixed-ligand complex by H(+) and X is possibly the rate-determining step, with the copper-phenanthroline complexes produced undergoing fast exchange with EDTA. The stability constant of CuRX(+) has been determined spectrophotometrically in 5% v/v dioxan at mu = 0.1, and at 25 degrees as [CuRX(+)]/[Cu(2+)] [R(-)] [X] = 10(21.2). The acceleration of the rate of substitution of copper (II)-PAN chelate may be explained by the fact that the Cu-PAN bond in the distorted octahedral mixed-ligand complex CuRX is weaker than in the reaction intermediate CuRY.