Low-salinity waterflooding has been identified as a cost-effective and environmentally friendly means to enhance oil recovery in carbonate reservoirs by decreasing hydrophobicity. Published work shows that a low concentration of 1-pentanol can further decrease the hydrophobicity, although the mechanism(s) remain unclear. In this work, we aimed to decipher the controlling factor(s), which prevail the process of wettability alteration, by adding alcohols in injected water. To achieve this aim, we examined the effect of alcohol carbon chain length and number of-OH functional groups on the contact angle of oil- brine carbonate using ethanol, isopropanol, 1-pentanol, and glycerol in high- and low-salinity brines. Moreover, to interpret the -contact angle results on the basis of a thermodynamic isotherm, we measured the zeta potential of brine-calcite and brine-oil with and without alcohols at ambient conditions. Contact angle results confirm that intermediate carbon chain alcohol (1-pentanol) shifts the wettability of the oil-brine-carbonate system to less oil-wet or more water-wet, implying greater hydrophilicity compared to other short carbon chain alcohols. Also, the number of-OH functional groups in an alcohol has a negligible effect on the contact angle and, thus, wettability alteration. However, the zeta potential at oil-brine and brine-calcite fails to explain the effect of alcohols on the wettability of the oil-brine-carbonate system, implying that the Derjaguin, Landau, Verwey, and Overbeek (DLVO) theory may not account for the effect of alcohol on the wettability alteration. We argue that increasing the length of the carbon chain likely increases-OH at oil-brine interfaces, which likely breaks the in situ bridges between oil and calcite surfaces, thus increasing hydrophilicity. Our finding shows that waterflooding efficiency may be boosted by adding 1-pentanol with a low concentration in high-salinity carbonate reservoirs, where conventional chemical-assisted enhanced oil recovery (e.g., polymer flooding and surfactant flooding) may not be viable.