A method of dispersing, dissolving, or reducing the viscosity of hydrocarbon foulants including heavy oil, tars, asphaltenes, polynuclear aromatic hydrocarbons, coke, polymers, light oil, oxidized hydrocarbon and thermal decomposition products, and the like in fluids in contact with hydrocarbon processing equipment comprising contacting the foulant with an effective amount of a halogen-free organic solvent having a density greater than water at the processing temperature.