Removal of naphthenic acid from acid oil with basic ionic liquids (ILs) of imidazole anion, 1-alkyl-3-methylimidazolium imidazolide, was investigated. A series of batch deacidification runs were carried out to evaluate the final naphthenic acids content of acidification samples after the exposure to agitation at distinct ionic liquid kinds and concentrations. Results showed that these ILs of high alkaline properties had excellent performance in deacidification, even processed high total acid number of oil, with the extremely low reagent/oil mass ratio. [EMIm]Im, [BMIm]Im, [HMIm]Im and [OMIm]Im (depending on the length of alkyl chain) were used to test the effects of the length of alkyl chain on deacidification. The longer the alkyl chain length was, the higher the naphthenic acids conversion would be. However, the mechanism of the process was inferred. In particular, when the reagent/oil mass ratio of [OMIm]Im-oil was 0.008, and the naphthenic acids conversion could be achieved 100%. The preliminary results reveal that the process saves time, consumes less energy and produces less pollution to environment. (C) 2012 Elsevier Ltd. All rights reserved.