High water-cut is a long-standing problem in the upstream petroleum industry. Typically one-fourth of the produced fluids from oil wells worldwide are hydrocarbons and the remaining is water. Self-selective in-situ gel formation is a new potential technology to decrease the production of water from oil reservoirs. In this method an oil-soluble chemical is being injected in the reservoir. The chemical, which in this case is tetra-methyl-ortho-silicate or tetramethoxysilane (TMOS) reacts with water and ultimately results in the formation of a semi-rigid gel in the water phase. Due to this gelation, the relative permeabilities of the formation to water and oil change in favor of the oil phase; therefore the ultimate effect of this gelation is a reduction of the water production rate from the reservoir. The subject of this paper was to model the flow of TMOS in a core, including the mass transfer of TMOS from oil phase to the water phase, and the occurring chemical reaction in the water phase.