Zeolite Y is a crystalline aluminum silicate used as an adsorbent matrix in the purification of proteins. Particle size enlargement is sometimes required and is achieved by compaction of the zeolite powder followed by heat treatment. To maintain a large surface area available for adsorption, additives are used in the pelletizing procedure. Ultrastable zeolite Y (USY) was compacted into pellets which were heat-treated at 800 or 1000 degrees C without affecting crystallinity. The samples were characterized by powder X-ray diffraction (XRD), electron microscopy, and N-2-adsorption/desorption. Ammonium acetate, used as an additive and sublimed off prior to the high temperature treatment, decreased the bulk density of the material, but did not affect the pore size distribution in the mesoporous range, i.e., 2-50 nm. Metakaolinite, which was used as a binder, affected the pore size distribution by splitting a broad local pore volume maximum at a pore diameter of 38 nm, observed in binderless ultrastable zeolite Y, into maxima at pore diameters of 20 and 80 nm. The volume of pores, within the diameter range of 2-5 nm, was decreased by 50%, probably due to a blocking effect of metakaolinite.