Crude oils are mixtures of a wide range of different molecules. Some of these molecules are amphiphilic and have acid and base functions with different pK(a). When a crude oil is in contact with a water phase, these acids and bases can be transferred from oil to water and become ionized. This transfer may affect the pH of the water depending upon parameters such as the acidity and basicity of the oil, the partition coefficients of the acids and bases, and the oil/water volume ratio. This paper reports on the evolution of the experimental pH of the aqueous phase after contact with a crude oil and various model oils (xylene containing fatty acids and fatty bases). We developed a model to quantify the influence of fatty acids and fatty bases on the final pH of the aqueous phase. The model is based on an average pKa for all of the fatty acids (pK(aA)) and another pK(a) for all of the fatty bases (pK(aB)). It was validated with the model oils and used to identify the key molecules responsible for mass transfer in the case of the studied crude oil.