18-Oxooctadecanoic acid is a derivative of long-chain fatty acids. It is specifically oxidized by P450BM-3 to form the corresponding α,ω-dicarboxylic acid (1,18-octadecanedioic acid) without undergoing a decarboxylation reaction that would produce an alkene with a carbon chain reduced by one carbon. 18-Oxooctadecanoic acid can be used to study metabolic pathways of long-chain fatty acids with aldehyde groups.