The exchange reactions between the ethylene glycol-bis(2-aminoethyl ether)-N,N,N′,N′-tetraacetic acid (EGTA) complexes of Fe(III), Co(II), and Zn(II) and 4-(2-pyridylazo)-resorcinol (PAR) were used to test the performance of partial least-squares regression to calibrate the kinetic measurement of binary and ternary metal ion mixtures using a stopped-flow injection system with a diode array detector. In approach mode 1, metal ion solutions were injected directly into the flow system, where their EGTA complexes were formed. In mode 2, metal-EGTA complexes were formed prior to flow injection. From data in mode 2, it was found that displacement of EGTA was pseudo first order with rate constants of 0.012, 0.016, and 0.078 s−1 for Fe(III), Co(II), and Zn(II), respectively. In mode 1, the rates of EGTA displacement from the cobalt and zinc complexes were the same but those for iron were not first order. In both modes, PLS calibration allowed satisfactory measurements of Fe(III), Co(II), and Zn(II). © 1994, American Chemical Society. All rights reserved.