Isotopic dating of diagenetic processes in relation to hydrocarbon charge in buried sedimentary sequences is of importance to the oil industry. Yun et al. recently published in the AAPG Bulletin (v. 94, no. 6, p. 759-771) on that topic a study entitled, "Dating Petroleum Emplacement by Illite Ar-40/Ar-39 Laser Stepwise Heating Technique." This study raises questions about basic aspects of Ar-40/Ar-39 dating fine-grained authigenic clay minerals, describes flawed methodology, and ignores important previous work. As isotopic dating of diagenetic processes may interest readers of the AAPG Bulletin that are not necessarily specialists in either isotopic geochronology or clay mineralogy, it appeared necessary to comment on both the technical aspects and the scientific interpretations of this study. We discuss especially the fact that the authors discarded Ar-39 recoil followed by an evaluation of its impact on the results and the interpretations.