The electromotive force E of the amalgam cell {KxHg1-x\KCl(m)\AgCl\Ag} has been measured as a function of the mole fraction x of K metal in amalgams and of the molality rn of KCl in (ethylene glycol + water), (acetonitrile + water), and (1,4-dioxane + water) solvent mixtures containing up to 0.8 mass fraction of the: organic component, at the temperature 298.15 K. The respective standard electromotive forces E-m(o) have been determined, together with the relevant activity coefficients y+/- as a function of KCl molality. A new scheme is here implemented for verification of the internal consistency of the y+/- results in terms of complementary pairs of concentration cells with transference. For interpolation purposes, the E-m(o) dependence on the mass fraction w Of the organic component of the solvent mixture within the range explored may be expressed by: E-m(o)/V = 2.1977 - 0.0872w(G) -0.0313w(G)(2), E-m(o)/V = 2.1978 - 0.1181w(A) + 0.0257w(A)(2) -0.217w(A)(3), E-m(o)/V = 2.1977 - 0.209w(D) + 0.274w(D)(2) -0.733w(D)(3), (the subscripts denoting G = ethylene glycol, A = acetonitrile, and D = 1,4-dioxane, respectively) which reproduce the observed E-m(o) values to within +/-0.3 mV. Analysis of the relevant primary medium effects upon KCl leads to a primary hydration number of 6.6 for KCl, in good agreement with previous results based on different methods.