Environmentally benign, non-catalytic, simple, and complete aromatic ring methylation of catechol derivatives by using 1,3,5-trioxane as the source of methyl groups was investigated in subcritical and supercritical water and under solvent-free conditions. Irrespective of the presence of subcritical and supercritical water as a reaction medium, catechol and 4-methylcatechol afforded the permethylation product 3,4,5,6-tetramethylcatechol. Only a small amount of 3,4,5,6-tetramethylcatechol (2% yield) was obtained under solvent-free conditions at 400 degrees C for 10 min. However, supercritical water considerably accelerated the formation of 3,45,6-tetramethylcatechol (13% yield) under the conditions of 400 degrees C 10 min, and 0.35 g/mL water density. (C) 2011 Elsevier B.V. All rights reserved.