The objective of this study was to develop a new measurement technique for the determination of pore size distributions (PSDs) of polymeric and ceramic membranes, including NF, UF, and MF membranes. The proposed method uses the fractional rejection (FR) concept of a solute in membrane pores. Experimental measurements were conducted using a high performance liquid chromatography (HPLC) equipped with size exclusion chromatography (SEC) columns and a refractive index (RI) detector. A specially designed membrane filtration unit was also used. Two different macromolecules, including nonionic polyethylene glycols (PEG) and natural organic matter (NOM) with ionizable functional (carboxylic and phenolic) groups, were used as solutes. Membrane PSDs, determined with PEG and NOM, can be defined as absolute and effective membrane PSDs, respectively. Two different types of membranes (flat-sheet polymeric and tubular ceramic) were used in this work. Experimental procedures include three major steps: (1) measurements of relative molecular mass (RMM) distributions of solutes included in the membrane feed and corresponding permeate, (2) the calculation of solute FR, and (3) PSD determination. The main results and advantages of this method are: (1) the PSD of various membranes with different pore sizes can be measured using a relatively easy method without significant limitations of pore size and membrane type; (2) various factors that affect membrane PSD, including pH, ionic strength, ion binding, and hydrodynamics, can also be evaluated; (3) the effective PSD of membranes with negatively-charged surfaces, and which exhibit significant shifts in PSD towards the lower RMM region can also be determined.