Ultrafiltration, UF, membranes positively-charged on a substrate of polyvinylidene fluoride, PVDF, have been modified by the deposition of a layer of poly(styrenesulfonate), PSS, which is negatively charged. These membranes have been treated by radiofrequency plasma at different powers and with different gases (argon, air and carbon dioxide). The membrane treated at 10.2 W with argon gave the best chromate retentions with good stability in water, alkaline and acid media. Surface zeta potential measurements confirmed a positive surface charge on the PVDF-substrate membranes, whereas those modified with PSS and argon-plasma treatment had a negative charge in the pH range 3-10. FTIR-ATR showed a true grafting of PSS on PVDF for the medium-power argon-treated membrane. SEM pictures of transversal sections confirmed continuity between the PVDF substrate, modified by the manufacturer, and the PSS layer. The chromium found on the modified membranes confirmed an electrostatically-determined retention. Pore-size distribution, as obtained by image analysis of SEM pictures of the surface, gave a slight reduction of pores but still within the ultrafiltration range in accordance with a non-size exclusion mechanism for retention. The modifications studied led to UF membranes provided with enough negative charges to boost retention of anionic species quite similar to those of nanofiltration membranes but with much lower applied pressures.