Previous reports of a UV spectral shift upon binding of the competitive inhibitor 19-nortestosterone (1) to 3-oxo-delta 5-steroid isomerase (KSI), coupled with UV resonance Raman results, have led to the conclusion that the enone moiety is polarized to a degree similar to that produced by complete protonation and that a proton relay may be involved in the enzymatic mechanism (Austin et al., 1992). These conclusions were partly based upon interpretations of the corresponding UV spectra of 1 in aqueous acid solutions. These interpretations are shown to be inconsistent with results of deuterium exchange studies and with spectra of model systems. Consequently, there is no evidence either for an extraordinary polarization of 1 produced by binding to the active site of KSI or for a proton relay mechanism.