The highly ortho-selective alkylation. of phenol with 2-propanol could be achieved without catalyst in supercritical. water at 673 K. The dehydration of 2-propanol gave propene and was followed by the reaction with phenol to produce 2-isopropylphenol as the main alkylphenol product. The ortho/para ratio of alkylphenols was above 20, and the yield of alkylphenols was 83.1%. Both the dehydration and the alkylation were accelerated with increasing water density, and this acceleration could not be attributed to the proton concentration alone. In supercritical water, the hydroxy group of phenol plays an important role for both dehydration and alkylation, and the reaction proceeds around the hydroxy group of phenol. In the dehydration of 2-propanol, phenol protonates 2-propanol and abstracts a hydrogen atom from 2-propanol. In the alkylation of phenol with propene, phenol provides a proton to propene and forms 2-isopropylphenol via an unsaturated ketone.