Abstract: A new method using a flow injection system with electrochemical detection was developed to determine acid values of fats and oils. VK3 (2-methyl-1,4-naphthoquinone) solution, i.e., ethanol containing 3 mM VK3 and 38 mM LiClO4, was used as the carrier solution. Flow signals were monitored at -0.33 V vs. Ag/AgCl. For preparation of a sample solution, an oil sample was completely dissolved in VK3 solution, or fatty acids were extracted from the sample into this solution. Aliquots (5 µL) of the sample solution were injected into the flow injection system. Acid values were determined based on flow signals for 14 samples and the results were found to be consistent with those by potentiometric titration. Relative standard deviation was less than 2%. Samples were processed at the rate of 60 h-1. The stability of fish and cod liver oils was followed by measuring acid values for 8 weeks. This method proved to be a simple and rapid means for acid value determination.