Solvent extraction experiments of Mn(II) by Cyanex 301 has been done from chloride solutions. The effect of variables, such as the initial pH and the concentration of Mn and extractant, on the extraction of Mn was investigated. The solvent extraction reaction was identified by applying a slope analysis method to the extraction data. In order to estimate the equilibrium constant for this solvent extraction reaction, the activity coefficients of aqueous species as well as the complex formation reactions between Mn and chloride ion were considered. Bromley equation was employed to calculate the activity coefficients. It was possible to predict the distribution coefficients of Mn from initial extraction data by employing the equilibrium constant. There was a good agreement between the experimentally measured distribution coefficients of Mn(II) and those predicted in this study.