The anti-Markovnikov alkylation of phenol with tert-butyl alcohol could be achieved without catalyst in supercritical water at 673 K. The dehydration of tert-butyl alcohol gave isobutene and was followed by the reaction with phenol to form 2-isobutylphenol as an anti-Markovnikov product, 2-tert-butylphenol and 4-tert-butylphenol. The hydroxy group probably participated in the anti-Markovnikov alkylation and the increase in water density enhanced the formation of 2-isobutylphenol as well as 4-tert-butylphenol.