Reverse osmosis (RO) and nanofiltration (NF) are pressure-driven membrane processes which can be used in wide range of industrial activities. Membrane characterization is necessary for connecting porosity with membrane separation properties. Pore size distribution (PSD curve) gives quantitative description of membranes’ pore size range and provides information about the particle size that can be retained. In this work the effect of pH value 4, 7 and 10 on PSD curves of commercially available RO (UTC-70HA, BW30, XLE) and NF (NF, NF90, NF270) membranes was studied. Values of permeate flux and retention factor of selected markers were tested, and from the experimentally obtained parameters with “SF-PF” model related PSD curves were calculated. Dense NF90 and RO membranes have unimodal PSD curves, with a maximum located in a range from 0.56 – 0.84 nm. Loose NF and NF270 membranes have bimodal PSD curves, with the majority of the pores located in a range from 0.72 – 0.82 nm, with an additional peak in a range from 1.30 – 2.34 nm. In comparison with pH 4, at pH values of 7 and 10, peaks of PSD curves are slightly moved towards the smaller pores (for UTC-70HA, XLE, NF and NF90 membrane), and/or exists a lower effective number of the pores (for BW30, XLE and NF270 membranes). It can be explained by narrowing of the pores at higher pH values, occurring due to electrostatic repulsion between homogeneous charged functional groups in a surface active layer.