The paper presents the theoretical foundation of the equation for the equilibrium constant concentration part logarithm without recourse to the activity coefficients. The result is based on the previously proposed quasichemical equilibrium model, on the diversity method of the system composition expression and on the experimental data analysis. The obtained five parameter equation of the fourth order of the components concentration has been employed in the mathematical treatment of the experimental evidence for the etherification reaction with the use of the uncertainty center technique.