The potential difference E of the amalgam cell {MxHg1-x\MCl or MCl2 (m)\ AgCl \Ag} (M=Rb, Cs, Sr, Ba) has been measured as a function of the mole fraction x(M) of M metal in amalgams and of the molality m of MCl (or MCl2) in (acetonitrile [A] + water [W]) solvent mixtures containing up to acetonitrile mass fraction w(A) = 0.50, at T = 298.15 K. The respective molal-scale standard potential differences E(m)degrees have been determined together with the relevant activity coefficients gamma(+/-) functions of the MCl (or MCl2) molality. The E(m)degrees dependence on the mole fraction x(A) of acetonitrile in the solvent mixture within the range explored turns out to be M linear for all the four metals M in the amalgams considered. Of course, also the difference ([E(m)degrees](W) - [E(m)degrees](A)), which is a measure of the primary medium effect upon transferring MCl (or MCl2) from pure water [W] to the acetonitrile [A] mixture, is linear in X-A. In this context, following Feakins and French's scheme, which implies volume fraction statistics, analysis of the relevant mol . dm(-3) scale primary medium effects, i.e., ([E(c)degrees](W) - [E(c)degrees](A)), upon MCl (or MCl2) as a linear function of the logarithm of water volume fraction, would lead to primary hydration numbers of 4.2 for RbCl, 4.0 for CsCl, 10.7 for SrCl2, and 10.3 for BaCl2, respectively, in acceptable agreement with literature data by Bockris based on different methods. (C) 2004 Elsevier Ltd. All rights reserved.