As the use of biofuels has increased in the last decade, there has been a level of concern over the effect that ethanol blends have on the material compatibility and operability of existing infrastructure. The focus of this research is to determine whether leak detection (LD) technologies are functioning properly in ethanol fuel blends. Fuels with different concentration of ethanol have different intrinsic properties. As new fuels with varying blends of ethanol emerge, the resulting variation in fuel properties might affect the functionality of LD technologies. Technology to detect leaks has been required since late 1989 when UST Operators were required to implement procedures to prevent and detect leaks in existing and USTs under Title 40 of the Code of Federal Regulations Part 280 (40 CFR 280) Technical Standards and Corrective Action Requirements for Owners and Operators of Underground Storage Tanks (Subpart D). When first employed, test procedures used to determine LD technology performance were commonly performed on USTs containing diesel fuel, in which the technologies tested generally behave in a similar manner as they do in gasoline. LD technologies tested with one of the procedures were then 'listed" by the National Work Group on Leak Detection Evaluation (NWGLDE)as having been evaluated by a third party in accordance with an approved leak detection protocol. Currently, the increasing desire to use motor fuels containing ethanol, such as #15 and flex Fuel (also Referred to as E85), has led EIPA, NWGLDE, and others to question the appropriateness of use of these LD technolgies with fuels that have different properties than the fuel on which they were originally tested and for which the test methods were designed.