Human placental aromatase is a cytochrome P-450 enzyme system which converts androgens to estrogens by three successive oxidative reactions. The first two steps have been shown to be hydroxylations at the androgen 19-carbon, but the third step remains unknown. A leading theory for the third step involves ferric peroxide attack on the 19-oxo group to produce a 19,19-hydroxyferric peroxide intermediate and subsequent collapse to estrogen. We had previously developed a nonenzymatic peroxide model reaction which was based on the above-mentioned theory, and we demonstrated the importance of 3-ketone enolization in facilitating aromatization. This study discusses the synthesis and nonenzymatic and enzymatic study of a 3-desoxy-2,4-diene-19-oxo androgen analogue. This compound was found to be a potent nonenzymatic model substrate and competitive inhibitor of aromatase (Ki = 73 nM). Furthermore, in an unprecedented event, this compound served as a substrate for aromatase, with conversion to the corresponding 3-desoxyestrogen.