Several polyoxometalates: Na2HPM12O40, H3PM12O40, Na2HPM12O40, (VO)H[PM12O40] and (n-Bu4N)(3) [PM12O40] (M=Mo and W) as well as (n-Bu4N)(3+x)[PW12-xVxO40] (x=0-3) were synthesized and characterized. Benzothiophene, dibenzothiophene and 4,6-dimethyl-dibenzothiophene were used as model sulfur compounds in gas oil. The oxidation reaction was performed using different polyoxometalates as catalyst and H2O2/acetic acid. The experimental results show that the W-based polyoxometalate catalysts are more active than the Mo catalysts. The oxidation reactivity of the catalysts depends on the type of countercation: Na+> H+>(VO)(+)>(n-Bu4N)(+). In a series of (n-Bu4N)(3+x) [PW12-xVxO40] (x=0-3) the order of catalytic activity is V-3 > V2 > V1 > V-0. The reactivity order of the sulfur compounds is: dibenzothiophene > 4.6-dimethyldibenzo-thiophene > benzothiophene. The catalytic system in this work was used for the oxidation of gas oil combined with solvent extraction to remove sulfur content in gas oil. Under mild reaction condition, high sulfur removal up to 98% can be achieved with high oil recovery (90%). (c) 2008 Elsevier B.V. All rights reserved.