Treatment of diatomite from the Shemakha deposit with aqueous hydrochloric acid and sodium hydroxide solutions yielded its modified forms studied in the process of regeneration of used (waste) compressor oil KM-40. It is shown that in cleaning capacity, the modified forms of diatomite are superior to natural diatomite. It is proved that acid and base centers occurring on the surface of the original and modified forms of diatomite are active in the used compressor oil cleaning process.