Abstract The nature of the complexes formed between Cu(II) and LIX 65N (H 2L) in ethanol has been studied by UV-visible spectrophotometry and the kinetics of the reaction between Cu(II) and H 2L by stopped-flow spectrophotometry at 25°C. The major complex, Cu(HL) 2, has a formation constant β 2 = [Cu(HL) 2]/[Cu][HL −] 2 of log β 2 = 26.5 l 2 mol −2 in 0.04 M NaClO 4. An additional complex, probably Cu(HL) +, can be detected in the absence of added NaClO 4. A third complex, identified as Cu 2(HL) 2 2+, was observed at higher copper concentrations with formation constant K d = [Cu 2(HL) 2]/[Cu(HL) 2][Cu] 1.7 × 10 4 l mol −1 in 0.04 M NaCIO 4. The rate of reaction of Cu(II) with H 2L to form Cu(HL) 2 obeys the rate equation, d[Cu(HL) 2]/d t = K rate[Cu] 1[H 2L] 1[H +] 0 where K rate has the value of 2.9 × 10 4 l mol −1 s -1 in the absence of added electrolytes.