A method of desulfurizing a petroleum liquid containing organosulfur molecules is disclosed. The method relies upon the use of an aqueous biocatalytic agent comprising, in preferred embodiments, a substantially cell free extract of a microorganism which functionally expresses an enzyme capable of selectively cleaving organic carbon-sulfur bonds even in sulfur-bearing heterocycles, wherein the extract contains a substantial proportion of the total activity of said enzyme expressed by the microorganism. An emulsion, preferably a microemulsion, is formed between this biocatalytic agent and the petroleum liquid. Reversible microemulsions are particularly preferred, due to facilitated recovery of a desulfurized petroleum liquid at the conclusion of treatment. The invention described is particularly well suited to the desulfurization of petroleum liquids having a high relative abundance of refractory organosulfur molecules, such as dibenzothiophene.