Methods of treating a petroleum hydrocarbon fluid are described wherein the petroleum hydrocarbon fluid is contacted with an asphaltene inhibitor composition having an ionic liquid and an asphaltene inhibitor. The ionic liquid has a cation of R1R2R3R4N+ or R1R2R3N+R8N+R5R6R7 and an anion, wherein R1, R2, R3, R4, R5, R6 and R7 are independently selected from hydrogen, a straight or branched C1-30 alkyl group, benzyl, a C7-30 alkylbenzyl group, a C7-30 arylalkyl group, a straight or branched C3-30 alkenyl group, a C1-30 hydroxyalkyl group, a C7-30 hydroxyalkylbenzyl group, an oxyalkylene or a polyoxyalkylene group or a zwitterion; R8 is a straight or branched C1-30 alkylene, an alkylene oxyalkylene or an alkylene polyoxyalkylene; and the anion includes halides, hydroxyl, bicarbonate, carbonate, alkyl carbonates, alkoxides, carboxylates, hydroxycarboxylates or a combination thereof.