Although investigation of the toxicological and physiological actions of alpha/beta-unsaturated 4-hydroxyalkenals has made great progress over the last 2 decades, understanding of the chemical mechanism of formation of 4-hydroxynonenal and related aldehydes has advanced much less. The aim of this review is to discuss mechanistic evidence for these non-enzymatic routes, especially of the underappreciated intermolecular pathways that involve dimerized and oligomerized fatty acid derivatives as key intermediates. These cross-molecular reactions of fatty acid peroxyls have also important implications for understanding of the basic initiation and propagation steps during lipid peroxidation and the nature of the products that arise.