A method for selective methylation of sulfides (particularly aliphatic sulfides) but not thiophenes in petroleum has been developed for positive-ion electrospray ionization (ESI) mass spectrometry. The new method uses only CH3I without AgBF4 to produce methyl sulfonium ions of sulfides for molecular composition analysis. It provides simplicity and saves time compared to a previously developed methylation method using CH3I/AgBF4 to methylate all sulfur compounds into ionic species analyzable by positive-ion ESI. The composition of sulfides by this method agrees with that of the sulfide fraction obtained from CH3I/AgBF4 methylation/demethylation. Hence, the separation of sulfides from thiophenes through elaborate procedures is eliminated. The new method was validated by model compounds and has been successfully applied to the analysis of sulfides in diesel, vacuum gas oil, crude oil, and atmospheric pressure distillation residue. When combined with the previously developed CH3I/AgBF4 method, the new method may have the potential to differentiate reactive sulfur, such as sulfides, from nonreactive sulfur, such as thiophenes. This could provide a means to develop correlations between sulfur types and corrosion, and possibly for other studies.