On December 30, 2013, the U.S. Environmental Protection Agency (EPA) published a final rule temporarily recognizing two different “Phase I” environmental site assessment (ESA) standards for commercial real estate and brownfield due diligence investigations. In a somewhat unusual administrative maneuver, the Final Rule amended EPA’s All Appropriate Inquiries regulations (AAI Rule) to reference two industry standards that may be used to comply with AAI requirements established under CERCLA, namely, ASTM E1527-05, issued in 2005 (the 2005 Standard), and ASTM E1527-13, issued in 2013 (the 2013 Standard). The Final Rule also indicated that reference to the 2005 Standard in the AAI Rule would eventually be deleted by way of a subsequent rulemaking. EPA published a proposal to accomplish that result in the Federal Register on June 17, 2014. During this interim period, users of Phase I ESAs had an option as to due diligence standards. As discussed, however, EPA’s failure to proceed in accordance with optimal administrative law principles may someday result in unexpected liabilities for parties who otherwise used the 2005 Standard in good faith.