Surfactant retention in reservoir rock is a major factor limiting effectiveness of oil recovery using microemulsion flooding processes. Effects of salinity and surfactant concentration on microemulsion phase behavior have a significant impact on relative magnitudes of retention attributable to adsorption vs. entrapment of immiscible microemulsion phases. Surfactant retention levels are determined by effluent sample analyses from microemulsion flow tests in Berea cores. Data for single surfactant systems containing NaCl only and multi-component surfactant systems containing monovalent and divalent cations are included. Retention is shown to increase linearly with salinity at low salt concentrations and depart from linearity with higher retentions above some critical salinity. This departure from linearity is further shown to correlate with formation of upper-phase microemulsions. Therefore, the linear trend is attributed to surfactant adsorption, and retention levels in excess of this trend are attributed to phase trapping. (28 refs.)