The potential difference E of the amalgam cell {CsxHg1-x\CsX (m)\AgX\Ag} (X = Cl, Br, I) has been measured as a function of the mole fraction x(Cs) of Cs metal in amalgams and of the molality in of CsX in (methanol + water), (acetonitrile + water), and (1,4-dioxane + water) solvent mixtures containing up to 0.75 mass fraction of the organic component, at the temperature 298.15 K. The respective standard molal potential differences E-m(o) have been determined together with the relevant activity coefficients gamma(+/-) as functions of the CsX molality. The found E-m(o) values show a parabolic decrease with increasing proportion of the organic component in the solvent mixture. Analysis of the relevant primary medium effects upon CsX shows that the CsX transfer from the standard state in water to the standard state in the (aqueous + organic) mixture is always unfavoured, and the acetonitrile is the least unfavoured co-solvent studied. Analysis of the primary medium effect upon CsI in terms of Feakins and French's theory leads to a primary hydration number close to zero, which is consistent with the results of supplementary EXAFS experiments on Cs+ and I- in (acetonitrile + water) solvent mixtures. (C) 2005 Elsevier Ltd. All rights reserved.