In situ ultraviolet absorption spectroscopy was applied to study the kinetics of N-alkylation of N-methyl-o-toluidine (NMOT) by supercritical (SC) methanol without any added catalysts. The pseudo-first-order rate constants of this reaction, estimated at temperatures between 250 and 300 degreesC and pressures between 5.4 and 30 MPa, were in the range of 1.8 x 10(-5)-1.5 x 10(-4) per s. The reaction apparent activation energy and volume were determined to be 104 kJ/mol and -500 cm(3)/mol, respectively. The results were also discussed comparing the alkylation of NMOT and other related compounds. Two possible N-alkylation mechanisms were proposed, which involved the formation of methanol hydrogen-bonded dimer or methanol autoprotolysis at high temperatures. (C) 2002 Elsevier Science B.V. All rights reserved.