The electrochemical behaviour of nitroprusside ion in potassium chloride aqueous-metha¬nol solutions was studied by both d.c. and rapid a.c. polarography. In the concentration range from 0 to 70 % (v/ v) methanol, the half-wave potentials of the three reduction waves were shifted to more negative values. The free energy of reduction was calculated and compared with the acceptor number of the mixed solvent. The free energy of reduction of [Fe(Cl\T)5NO ]-2 to [Fe(CN)5NO]~3 in the mixed solvent is directly proportional to Gutmann’s acceptor number of the media. The effect of potassium chloride concentration on the half-wave potential, the thickness of diffuse layer and the values of the rate constant for the third wave of sodium nitroprusside was discussed.