4-HNE (4-hydroxy-2-nonenal) is a highly reactive α,β-unsaturated aldehyde generated from oxidation of polyunsaturated fatty acids and has been suggested to play a role in the pathogenesis of several diseases. 4-HNE can bind to amino acids, proteins, polynucleotides, and lipids and exert cytotoxicity. 4-HNE forms adducts (Michael adducts) with cysteine, lysine, as well as histidine on proteins with the thiol function as the most reactive nucleophilic moiety. Thus, detoxification strategies by 4-HNE scavenging compounds might be of interest. Recently, hydrogen sulfide (H2S) has been identified as an endogenous vascular gasotransmitter and neuromodulator. Assuming that the low-molecular thiol H2S may react with 4-HNE, methods to monitor the ability of H2S to counteract the protein-modifying and cytotoxic activity of 4-HNE are described in this chapter.