The separation of mercury from aqueous solutions by complexation-ultrafiltration was investigated. Polyethylenimine (PEI) was used as polymeric complexing agent. Effects of pH and chloride ions concentration on mercury retention were studied. Mercury retention depends strongly on both pH and chloride ions concentration. For neutral solutions mercury retention values close to 100% were obtained. A process allowing mercury separation and concentration was studied. It couples two ultrafiltration stages: the first one concentrates mercury/PEI complex and the second one enables mercury separation and polymer recycling. This process was experimentally tested. Results showed that it is possible to concentrate mercury ten fold while a permeate with very low mercury concentration is collected. Recycling of PEI does not modify operation performance.