The compounds of fresh vegetable oils usually show a non-polar character. While frying, the compounds showing a polar character are produced, and they decrease the quality and healthful properties of a given frying oil. In order to improve the quality of frying oil, it can be purified using adsorbents of various kinds. Consequently, investigations have been launched to study the technique of adsorptive purification of rapeseed oil used to fry food products. This purification technique involved activated carbons with modified surfaces. The activated carbon is characterised by a low content of functional groups of a polar character. Thus, it is necessary to modify its surface for the purpose of increasing the efficiency of the impact exerted by the activated carbon. The modification procedure consists in increasing the number of functional groups of a polar character. Owing to this modification, it is possible to produce an adsorbent that may be useful in removing polar compounds contained in the used frying oil. Activated carbons having modified surfaces were developed; the modification of their surfaces was performed by oxidizing initial carbons with a nitric acid solution, a hydrogen peroxide solution, and a sulphur acid. After the carbon surface modification process accomplished, the activated carbons with modified surfaces significantly improved the quality of oils applied to fry food products. The best effect of adsorbing free fatty acids was noted in the case of activated carbon oxidized using a hydrogen peroxide (ARP). The highest decrease in the content of triacyloglycerol oxidation products was reported when the activated carbon oxidized by a dissolved nitric acid was applied. And the activated carbon oxidized by a concentrated sulphur acid caused an essential increase in the ratio of unsaturated fatty acids to saturated fatty acids.