We report successful alkylation reactions in near-critical water in the absence of added acid catalysts. Water has a strong tendency to ionize as the temperature is increased, allowing water in the near-critical region (250-350 degrees C) to act as an effective acid catalyst. By simultaneously employing near-critical water as the reaction solvent and catalyst, the need for environmentally hazardous organic solvents and acid catalysts is eliminated. The high hydronium ion content of near-critical water proved to be sufficient to synthesize a variety of substituted phenols with tertiary, secondary, and even primary alcohols. The reaction products for each of these reactions are reported.