This paper described that there were two or more endpoints in the curve when potentiometeric titration was used to determine the contents of mercaptan sulfur in residue fluidized catalytic cracking (RFCC) gasoline. Comparing with the one endpoint of a mixture solution of C-2-SH to C-10-SH determined under the same conditions, we found some components in RFCC gasoline affected the result of titration. A facile method washing the gasoline with 2 wt.% HCl solution was brought out to eliminate the interfering components from the RFCC gasoline. After being treated with this method, all RFCC gasoline that contained 1427 mug/ml mercaptan sulfur (S-RSH) met the quality specification (SRSH shall not exceed 10 mug/ml) [GB 17930-1999, Chinese national standard for unleaded petrol (gasoline) for motor vehicles]. The analysis of gas chromatography-mass spectrometry (GC-MS) and gas chromatography-infrared spectrometer (GC-IR) verified that the interfering components were aniline, phenol and their alkyl-substituted derivatives. (C) 2003 Elsevier B.V All rights reserved.