The prediction and prevention of both sodium and calcium naphthenate "scales" is an important issue in oil production. A broad description of how these scales form has been available for some time, although most experimental findings are still of a qualitative nature. In this paper, an equilibrium thermodynamic model is presented for predicting naphthenate partitioning and precipitation in an oil/brine immiscible system from some chosen initial conditions (i.e., naphthenate initial concentration in oil, brine pH, [Ca2+], etc.). This model has, with some assumptions, been applied to both model and real naphthenate system. This model describes two types of naphthenate experiment: 1) full naphthenate precipitation, and 2) simpler "pH change" experiments in which no precipitation occurs. To predict naphthenate precipitation, the theory suggests knowing: 1) the partition coefficient of the naphthenic acid, HA, between the oil and the water phases, Kow; 2) the pKa of the naphthenic acid in water; and 3) the solubility product, KCaA2 (or other similar solubility parameter), of the naphthenate deposit. In the simpler pH change experiments, only the first two of these parameters (i.e., Kow and pKa) are required. Using the naphthenate model without precipitation, the effect of varying parameters on the degree of pH change predicted at equilibrium in the oil/naphthenic acid/brine system was studied. Also, the model was used to examine the sensitivities of the various parameters on the final pH was also applied. The comparison between the model predictions and experiment at a higher brine pH value is overall satisfactory. © 2009 Society of Petroleum Engineers.