Abstract The title subject has been studied through stationary and transient polarization measurements (combined with stirring and ring-disc experiments) on the Ni/Ni(II) electrode and equilibrium measurements on the Ag/AgCl electrode at 25°C. The Ni/Ni(II) electrode is found to react in two consecutive steps with Ni(I) as intermediate. The ion-transfer step Ni/Ni(I) is chloride independent and seems mostly to occur directly at kink sites. The electron-transfer step Ni(I)/Ni(II) involves parallel transfer by the couples Ni(H 2O) + 6/Ni(H 2O) 2+ 6 and NiCl(H 2O) 5/NiCl(H 2O) + 5 and is independent of kink sites. No clear effect or sign is seen of possibly adsorbed chloride ions. Data are obtained for the mean ionic activity coefficient of NiCl 2 in the mixed solutions applied. An attempt is made to determine single-ion activities by combining kinetic and thermodynamic data.