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Carbonate reservoirs become more water-wet during thermal recovery. The effect of temperature on wettability-altering process is caused by contribution from several parameters involving fluid/fluid and fluid/rock interactions. This paper aims at describing the interrelationship between different parameters of a simple oil/water/rock model over temperature range of 25 to 130 degree centigrade. Saturated and unsaturated fatty acids as well as naphthenic acids with saturated and unsaturated rings are selected for this work to alter the water-wet calcite surface. The type of selected acids is based on the distribution of these components in reservoirs in the Norwegian continental shelf. Contact angle measurements on the treated calcite surfaces are used as indication of wattability alteration. At fluid/fluid interface the interfacial tension and distribution of the solutions of n-decane /fatty acids /water systems are measured at elevated temperature. A set of experiments is also performed in order to understand the role of the temperature on fluid/rock interface by zeta potential measurements. As the temperature increases, calcite surface becomes more water-wet. The obtained results at fluid/fluid interface (IFT and distribution coefficients) and contact angle measurements show that the trend of decrease in contact angles with temperature follows the same trend as IFT and distribution coefficients, specifically if one divides acids to saturated and unsaturated separately. Electro-kinetic measurements (zeta potential) of calcite surfaces with temperature demonstrate that increasing temperature reduces surface charge to less positive, which may enhance the repulsive forces between dissociated acids and calcite surface. Due to this change in surface charge, the adsorption of acids on the surface becomes less effective at high temperatures; hence wettability of the calcite surface tends to be more water-wet. Introduction The wettability of a hydrocarbon reservoir depends on how and to what extent organic components are adsorbed to the solid phase's present.[1] For carbonate reservoirs naphthenic acids and number of carboxylic acids are recognized to be the most frequent acidic components that adsorbed on the surface and altered the wettability.[2–5] The degree to which the wettability is altered by these components is determined by several parameters. Temperature is one of those controlling parameter that has an effect on both oil/water and water/mineral interfaces. Many authors have reported a shift in wettability of mineral surfaces toward water-wet at elevated temperatures.[6–9] Increasing the solubility of adsorbed materials from surfaces and decreasing the IFT are two different effects of temperature on wettability at elevated temperature.[2] Several work have directed to the partition coefficients of carboxylic acids between oil and water phase as a function of pH and salinity and to perhaps a lesser extent on the effect of temperature.[10–13]. Hamouda et al.[13] have performed an extensive experimental work on wettability alteration of calcite surfaces due to dissolved carboxylic acids in oil phase at ambient temperature and different pHs. It was shown that there is a possible implication between change in IFT and partitioning with the wettability of the calcite surfaces. They showed that the high soluble acids in water owing low partitioning coefficients hence lesser effect on IFT has minor change on wettability alteration of calcite surfaces. Increasing pH decreased the IFT between water/n-decane /fatty acid systems as well as partition coefficients of acids from oil to the water phase. Consequence of those changes resulted in decrease in contact angles on calcite surfaces. These behaviors were explained by possible increase in the repulsive forces due to dissociation of acids at water/n-decane interface hence change in the surface charge of calcite surface. Depending on the oil composition, both decreasing and in some cases increasing in IFT with the temperature were reported in literatures.[6–7,14] In terms of partition coefficients it has been shown insignificant effect by the temperature.[11,15]