We report an experimental study of surfactant-enhanced alkaline/diluted heavy oil systems to understand the combined effect of different parameters, such as pH, salinity, surfactant type, and concentration, on diluted heavy oil/brine phase interfacial behavior. The interfacial tension (IFT) was measured using pendant drop and spinning drop tensiometers. Different surfactants [Triton X405, sodium dodecyl sulfonic acid (SDS), and sodium dodecylbenzene sulfonate (SDBS)] were tested. SDBS was by far the most efficient surfactant, reducing the IFT to ultra-low values of similar to 4 x 10(-4) mN/m at a concentration of only 0.05% and a pH of 11. This efficiency is due to a synergistic effect between the in situ surfactant (produced by saponification of the acid groups present in the crude oil) and the added surfactant (SDBS).