To improve fouling resistance, polyamide reverse osmosis (XLE) and nanofiltration (NF90) membranes were modified by grafting poly(ethylene glycol) (PEG) diglycidyl ether (PEGDE) to their top surfaces from aqueous solution. The effect of PEG molecular weight (200 vs. 1000) and treatment solution concentration (1% (w/w) vs. 15% (w/w)) on water flux and NaCl rejection was measured. PEGDE grafting density as well as surface properties of modified and unmodified membranes, including charge, hydrophilicity and roughness, were measured and compared. The fouling resistance of modified membranes to charged surfactants (i.e., sodium dodecyl sulfate (SDS) and dodecyltrimethylammonium bromide (DTAB)) and emulsions of n-decane and these charged surfactants was compared to that of unmodified membranes. In general, modified membranes exhibited improved fouling resistance and an improved ability to be cleaned after fouling compared to unmodified membranes. Fouling resistance increased with increasing PEG molecular weight, but showed little dependence on treatment solution concentration, suggesting that further improvements in membrane fouling resistance might be obtained by using lower concentrations of higher molecular weight PEG for surface modification.