Surfactants have a wide range of applications: in addition to their industrial use as detergents, being at the same time a serious source of drinking water and waste pollution, they play an important role in analytical and preparatory chemistry and biology as a means to reduce surface tension. An example of the precipitation phenomenon is laundry detergents, in which anionic surfactants provide cleaning, and cationic surfactants offer fabric softening/antistatic properties. A thermodynamic model for establishing the domains of phase stability in mixtures of anionic and amphoteric surfactants for a wide range of pH variation and various initial chemical compositions of the analyzed heterogeneous mixtures has been developed. A mixture of anionic and surfactants, consisting of three components at intermediate pH levels: anionic, cationic (protonated amphoteric) and zwitterionic (non-protonated amphoteric) was studied. Knowledge of the composition of each surfactant in both monomer and micellar forms (monomer-micelle equilibrium) and in the form of a precipitate is important in applications using this mixture. The systems studied in this paper contain sodium dodecyl sulfate (SDS) and dimethyldodecylamine oxide (DDAO+), which are used in a wide range of consumer products. The pH value has been shown to have a substantial effect on the precipitation of the surfactant mixture when the loading of the surfactant depends on the pH of the medium. The concentration of charged monomers depends on the pH due to the reactions of the surfactant with hydroxyl and hydrogen ions. Dimethyldodecylamine oxide (DDAO+) is an amphoteric surfactant that may exist as a cationic or zwitterionic surfactant depending on the pH of the solution. The amine group of the surfactant DDAO can protonate in a similar manner to weak acids. By combining the use of the precipitate solubility product constant and the dissociation/protonation constant within the developed approach, taking into account all side reactions, a thermodynamic model was presented and used to predict the solid phase existence conditions. All the global Gibbs energy variation calculations were performed for the concentrations of both surfactants lower than their CMC values. Precipitation of mixtures of dimethyldodecylamine oxide and sodium dodecyl sulfate has been investigated at different pH levels. The protonated form of DDAO+ carries a positive charge and precipitates with the opposite charged DS-. The precipitation areas of the compound DDAO+-DS-(S) become narrower as the surfactant concentrations decrease. At the same time, when the concentrations of both surfactants decrease, the pH of the coexistence of the two solid phases shifts to its higher values, from a more acidic environment to a neutral one. The ΔG(pH) curve passes through a maximum, then decreases, due to the deprotonation of DDAO+ and the formation of the neutral zwitterionic form of dimethyldodecylamine oxide, DDAO0. The developed thermodynamic model is in agreement with the existing experimental data at different pH levels. The current study provides guidance for handling the precipitation process, as well as a perspective on the entra-chemical phenomena of precipitation and micelle formation in the investigated systems.