The paper deals with diafiltration of a solution of PAE resin, the purpose of which is to lower the concentration of residuals DCP and MCPD. Nanofiltration membranes Koch SR3D and Koch SelRO MPS-34, both with MWCO of 200 daltons are used. Short term an long term changes in permeability of membranes, as well as rejection of DCP and MCPD are recorded. Filtration is taking place at neutral pH. As expected, results show that PAE resin does not permeate through the membrane. More unexpected is the fact that DCP and MCPD do not permeate through the membrane freely. Results show there is no membrane fouling during the operation, but the membrane fouls when the device is stationary. Change of pH both towards the acidic, as well as caustic environment, contributes to faster membrane fouling. We discuss mechanisms of rejection of DCP and MCPD and mechanisms of membrane fouling.