- Abstract Increased protein fouling of polyether sulphone membranes after NaOCl cleaning was previously reported but not explained. Here we show that the cleaning increases the hydrophilicity, and the degree of increase linearly correlates with the amount of adsorbed protein. The high initial flux through the cleaned membrane is a result of the hydrophilization of the membrane surface and a promise for the enhanced fouling. We propose that the proper oxidative cleaning should target the restoration of the initial flux and not its increase over initial values. The previously reported pore size changes are subjective as higher hydrophilicity of the membrane surface increases water permeability and adsorption of size test solutes.