Waste cooking oil can be recovered and processed into animal feed additives after purification. Unfortunately, the traditional purification processes are insufficient for the removal of the harmful compounds formed during frying, mainly about malondialdehyde and other 2-thiobarbituric acid reactive substances. In the present paper, firstly, a simple and reliable HPLC method was developed to measure the content of malondialdehyde in purified waste cooking oil. The detection limit and the standard recovery of the analysis method are 1.20 x 10(-5) g l(-1) and 96.5-99.2%, respectively, which is accurate and valid enough for the detection of malondialdehyde in waste cooking oil. Furthermore, the removal of malondialdehyde and other 2-thiobarbituric acid reactive substances from waste cooking oil was investigated using three methods, i.e., water extraction, physical adsorption, and chemical adsorption. Results show that among the three methods, chemical adsorption using lysine or monosodium glutamate as chemical adsorbent is the most effective, which can remove 80% of malondialdehyde and other 2-thiobarbituric acid reactive substances from waste cooking oil. (C) 2011 Elsevier Ltd. All rights reserved.