Drill cuttings are cleaned of oil-based drilling mud and oil using a cleansing agent, such as a C6-C10 carboxylic acid, that is preferentially oil soluble at one pH and preferentially water soluble at another pH. The oily cuttings are treated with the carboxylic acid and then washed with an aqueous, alkaline washing solution, which converts the carboxylic acid to its water soluble salt and, with vigorous agitation, lifts most of the oil from the cuttings. In other embodiments, the process is used for cleaning oil-contaminated soil, sand, and gravel, and for separating the oils from the solids in oily sludges.