The acidic ionization (hydrolysis) constant of ferrous ion has been measured in the range of temperature between 20 and 40°C and of ferrous ion concentration between 0·0016 and 0·062 M, in 0·5 to 2 M sodium perchlorate solution, according to the theory of the constant ionic medium. The value of pKa′ obtained at 25°C is 6·74 ± 0·06. Within experimental error Ka′ is independent of the ferrous ion and sodium perchlorate concentrations. The temperature dependence of Ka′ is expressed by pKa′=2625T-2-037. The increase of enthalpy and free energy of the hydrolysis reaction are: ΔH_298° = 12,000 ± 1,000 cal/mol; ΔG_298° = 9,165 ± 100 cal/mol. The information has been used to draw conclusions on the character of the ferrous ion in aqueous solution; on its amphoteric nature; and for checking the solubility data of ferrous hydroxide.Instituto de Investigaciones Fisicoquímicas Teóricas y Aplicadas