At petroleum refining facilities with a long operational history, it is likely that some products were released to the subsurface and migrated to the water table. At or near the water table, these products might have commingled with a pre-existing light non-aqueous phase liquid (LNAPL) plume(s). Depending on the types of products involved and site hydrodynamics, commingling might result in the formation of a “new” LNAPL that exhibits similar characteristics to products that were manufactured via intentional blending by refinery operations. This study presents a case in which subsurface commingling of two intermediate gasoline-range products occurred at a petroleum refinery. The commingled “new” product appears almost identical to finished gasoline. As the intermediate stream products are typically sourced from refinery and finished products from either refinery or other sources (e.g., pipeline corridors), distinction of the commingled gasoline intermediate stream product from finished gasoline becomes critical not only for resolving liability issues, but also development of a remedial strategy. In this study, the source relationship between the gasoline-range intermediate stream product and finished gasoline was resolved using multiple lines of evidence including a gasoline additive, LNAPL chromatograms, diagnostic compounds (biomarkers) and ratios, and site LNAPL hydrodynamics.