Methylation reaction of indole was studied in supercritical methanol (350 degrees C, 215 atm) catalyzed by TMOS and TEOS. Use of both orthosilicates shows the same result of indole methylation. The indole conversion is similar to 30% for 5 h of the reaction with and without catalysts. But catalytic reaction is more high-selective on 3-methyl-1H-indole formation - 95-96 mol% against 60 mol% for non-catalytic methylation. Indole conversion in catalytic reaction increases by a quarter (with no change of selectivity) when two equivalents of water are added into reaction mixture. Methylating system was assumed to be the TMOS-Me0H, and methylation proceeds via cyclic, multi-centered transition state (TS). The TS involves molecules of indole, TMOS, methanol, and water (if used), which are orientated in a certain way to each other by hydrogen and coordinate bonding. The reaction does not occur in supercritical ethanol or isopropanol with and without using the catalysts. (C) 2012 Elsevier B.V. All rights reserved.