A new approach to modelling of some binary (hydro-organic, HL/H(2)O+B) acid-base systems with organic co-solvent B fully miscible with water, is suggested and applied for the determination of acidity parameters pK for some weak acids HL. The models are designed to get the pK=pK(x) relationships (x-mole fraction of B in H(2)O+B) from results of pH titrations made in such systems and for the determination of pK(B)=pK(1) for HL in pure B. The Redlich-Kister equation, together with its asymmetric extensions, and the Legendre functions with orthogonal polynomials, appeared to be suitable for such purposes.