The first acidic ionization constant of Co(II) as cobaltous perchlorate has been determined by a potentiometric titration in a sodium perchlorate medium of constant ionic strength at temperatures ranging from 15 to 40°C and at different concentrations of Co(II) from 2.5 to 25 × 10−3, M and total ionic strength from 0·327 to 0·867. The assembled information indicates the hydrolysis reaction of this ion is a mononuclear reaction where the complex CoOH+ is formed with ΔG_298° = 13·4 ± 0·2 kcal/mole and ΔH_298° = 8·2 ± 2·0 kcal/mole. The second acidic ionization constant is higher than the first. This shows that a disproportionation reaction of CoOH+ ions is thermodymanically favoured and has to be considered in the ionic equilibria of aqueous Co(II) ions.Instituto de Investigaciones Fisicoquímicas Teóricas y Aplicadas