In this study, we investigated and compared the performance (permeate flux, solute rejection and membrane fouling) of an ultrafiltration membrane and a nanofiltration membrane with water and formamide as solvent. The dextran solutions with water as solvent had lower electrical conductivity and usually greater surface charge compared to that in formamide, which facilitated the electrostatic interactions taken place. As a result, faster flux decline and greater fouling resistance were usually observed in water-based solutions in both UF and NF with the same dextran used as solute. In UF, the solute rejection usually increased as the solvent switched from formamide to water, since stronger electrostatic interactions tended to impede the transport of solute through membrane. Opposite trend was found with NF, that formamide usually led to higher solute rejection compared to water as solvent. Despite the differences of surface charge altered the membrane performance prominently, no quantitative correlation was found between permeate flux and zeta potential values. The analysis using a modified Hermia's pore blocking model indicates that the fouling mechanisms could be altered as the change of solute to membrane pore size ratio, surface charge of membranes and solutes, as well as the solvent properties. [ABSTRACT FROM AUTHOR]