S-Methylsulfonium salts, obtained by a new desulfurization process for light oil based on methylation of sulfur-containing compounds using CH3I and AgBF4, were used as a methyl-transfer agent for transmethylation of 2-methyl-2-octene. The sulfonium salts obtained from light oil showed higher transmethylation activity than the salts of dibenzothiophene (DBT) and benzothiophene (BT), used as model sulfonium salt. The salts obtained from light oil are derived from highly alkyl-substituted DBTs and BTs, of low nucleophilicity (low electron density on the sulfur atom), and release the methyl group easily, thus giving the higher transmethylation activity. The product distribution was almost the same as that obtained by the model sulfonium salts, suggesting that the salts obtained from light oil are a potential effective methyl-transfer agent. (C) 2003 Elsevier B.V. All rights reserved.