A new method to remove the nitrogen compounds from lubricating base stocks was revealed by using the complexing and crystallization of oxalic acid and nitrogen compounds in this paper. The data indicated that the reaction and filtration temperatures obviously influence the nitrogen removal efficiency. The suitable reaction temperature was 100-130degreesC and the reaction time is more than 20 min. The crystal water in oxalic acid can lower the removal efficiency, which can be improved when inert gas is introduced to take out the crystal water in oxalic acid. When clay was used in the reaction system, the nitrogen removal efficiency was improved and the filtration temperature was also elevated. Oxalic acid can remove more than 98% of basic nitrogen compounds and more than 70% of nonbasic nitrogen compounds from the lubricating base stocks. The removal of basic nitrogen showed a linear relationship with the removal of nonbasic nitrogen. (C) 2004 Published by Elsevier B.V.