The ultrafiltration membrane rejection capability is most often characterized by molecular weight cutoff (MWCO). The value is found by rejection of organic solutes and the evaluation of particle retention requires a conversion of either MWCO to pore size or particle diameter to molecular weight. The conversion affects the accuracy of reported values and results in a gap between reported and measured retentions. We suggest a novel, simple and effective pore size test based on synthesis and membrane transfer of rigid nanoparticles. Gold and silver 3-50 nm monodispersions had delivered a comprehensive pore size distribution including d(100), a pore diameter for which a membrane has a 100% retention capability. The maximal pore size in UF membrane structure can hardly be detected with other methods although it is much needed for precise separation analysis. The d(100) values in tested UF membranes vary between 40 nm and 50 nm depending on the membrane material. The polymer membranes are more flexible than the ceramics and their d(100) is usually much higher than MWCO. The d(100) increases with high transmembrane pressure or after oxidative chemical cleaning. For some membranes the d(100) values can be correlated with d(90) but not with d(50). (C) 2011 Elsevier B.V. All rights reserved.