The paper presents a critical analysis of the accuracy of the system non-ideality description for chemical equilibria calculation in diisopropyl ether synthesis. Depending on the process parameters, the analysis refers to the reactions in the gas or the liquid phase. Eight different approaches based on equations of state are considered. The first four use classical mixing rules with or without the binary interaction coefficients k(ij). The last four apply the mixing rules combining equations of state and excess Gibbs free energy models. The results of calculations are compared with the experimental ones.