Separation of mercury from aqueous solutions by continuous polymer-enhanced ultrafiltration (PEUF) was investigated. Polyethyleneimine (PEI) was added to the solutions as the complexing agent before circulating the solution in a laboratory-scale continuous ultrafiltration system. Effects of mercury-to-polymer ratio, pressure drop and feed solution circulation rate on retention of mercury and permeate flux were studied. Constant retention values close to unity were obtained until a critical mercury-to-polymer ratio was exceeded. For higher metal loadings, retention values decreased linearly. Permeate flux increased linearly with pressure drop and was not affected by other parameters such as metal loading and feed circulation rate. Therefore, concentration polarization was not a problem for the studied range of variables.