Low-salinity-water injection (LSWI) is an emerging way to improve waterflood performance at low cost under certain conditions. The effect of LSWI on the oil recovery from carbonate formations has been well-documented in the laboratory and to a limited extent in the field. In this study, a mechanistic geochemical model is proposed that addresses the effect on oil recovery of different geochemical reactions resulting from LSWI. The proposed model was used to history match recently published corefloods by use of the UTCHEM (2000) mechanistic reservoir simulator. Moreover, other sets of corefloods were chosen to validate the proposed LSWI mechanistic model. The geochemical model in the UTCHEM simulator was modified to calculate the molar Gibbs free energy of the brine. In the proposed LSWI mechanistic model, the relative permeability curves, including the residual oil saturation (ROS), are functions of the molar Gibbs free energy. Coreflood experiments were simulated and history matched by use of the proposed model. The proposed LSWI mechanistic model was further extended to include weakly oil-wet to mixed-wet carbonate rocks. The mechanistic model captures the effects on oil recovery of both wettability alteration and/or dissolution/fines migration through changes in the molar Gibbs free energy. This model can now be used for oil-recovery predictions and optimization of LSWI field applications.