In the study, particle size distribution of the MIEX® resin was presented. Such analyses enable to determinate whether presence of fine resin fraction may be the reason for unfavorable membrane blocking during water purification by the hybrid MIEX®DOC – microfiltration/ultrafiltration systems. Granulometric analysis of resin grains using the laser diffraction particle size analyzer (laser granulometer) was carried out as well as the microscopic analysis with scanning electron microscope. The following samples were analyzed: samples of fresh resin (a fresh resin – not used in water treatment processes) and samples of repeatedly used/regenerated resin that were collected to analysis during mixing and after sedimentation process. Particle size distribution was slightly different for fresh resin and for repeatedly used/regenerated resin. The grains sizes of fresh resin reached approximately 60 μm (d10), 120 μm (d50) and 220 μm (d90). Whereas the sizes of repeatedly used/regenerated resin were about 15 μm (d10), 40 μm (d50) and 115-130 μm (d90). The smallest resin grains sizes were in the range of 0.3-0.45 μm. This ensures that the ultrafiltration membranes retain all resin grains, even the smallest ones. Whereas the microfiltration membranes must be appropriately selected to guarantee full separation of the resin grains and at the same time to exclude a membrane pores blocking.