A thermodynamic model has been developed and used to predict the precipitation conditions using the solubility product relationship between surfactant monomer concentrations, in order to calculate the monomer-precipitate equilibrium, in the mixtures of anionic and cationic surfactants for a wide range of pH variation and various initial chemical compositions of the studied heterogeneous mixtures. The developed model could predict the amount of precipitate that formed in the mixture of cationic and anionic surfactants, the results proving to be in agreement with the experimental measurements. Mixture of sodium dodecyl sulfate (SDS) and dodecylpyridinium chloride (DPCl) has been shown to react in solution with the formation of a precipitate in a wide range of concentrations. When the micelles are not present in the solution, this reaction can be modeled by a simple solubility product between the total surfactant concentrations. In addition, the developed approach provides an explicit equation which predicts the amount of solid phase which forms in any NaDS-DPCl mixture, so that surfactant equilibrium concentrations (after precipitation) can be determined. The chemical composition in each monomer phase, which is determined in the computerized calculation process, helps to understand and predict the solution compositions corresponding to the precipitation limits of anionic/cationic surfactants. The results of the calculation of the global Gibbs energy variations of this system for different chemical compositions show that with the decrease of the total concentrations of the surfactants the areas of existence of the solid phase, DSDP(S), narrows. For the concentrations C(DS-) = C(DP+) = 1 ∙ 10-3 mol∙L-1 the precipitate DSDP(S) exists in the whole studied range of pH values. From the obtained data it is possible to determine the pH of the coexistence of two solid phases, lauric acid HDS(S) and dodecylpyridinium dodecyl sulfate DSDP(S). Thus, conditions can be predicted in which there is only one phase or when the precipitation of one or both surfactants can be avoided. The obtained results are in agreement with the existing experimental measurements.