S-methyltetrafluoroborate salts of the thiophenic compounds (CH3-S+:BF4-) present in LCO petroleum fractions were obtained and analyzed by H-1 and C-13 NMR spectroscopy. The methylation of the samples was carried out using 99.5% C-13 enriched methyl iodine, to improve the sensitivity of the technique. The amount of the methylated derivatives was determined by the internal standard method; using dioxane as a reference, 37 sulphur compounds were detected. Among them, benzo[b]thiophene, dibenzo[b,d]thiophene, and several isomers of methyl, dimethyl and trimethyl[b]benzothiophenes were the most abundant. With this research, it was demonstrated that NMR spectroscopy can be used to analyze thiophenic compounds from petroleum medium fractions. (C) 2004 Published by Elsevier Ltd.