The behaviour and mechanism of extraction of platinum (IV) with unsymmetrical sulfoxide BSO from HCl media are studied. The results showed that the percentage extraction of Pt (IV) achieved 99.55% from 4 mol·L-1 HCl media with 0.5 mol·L-1 BSO-kerosene as an extractant. This was an exothermic reaction with an enthalpy of –12.35 kJ·mol-1. The structure of complex was discussed by slope analysis method, UV spectra, FT-IR spectra. The extraction mechanism under the above mentioned conditions could be determined to be an ion-pair formation with the protonated BSO, and the composition of complex may be [2(BSOH+)]·[PtCl62-]·[BSO·HCl]·[2BSO](0).