Porasil porous silica has been characterized by the methods of gas adsorption–desorption isotherms, mercury porosimetry, and electron microscopy. The pore structure has been shown to be quite heterogeneous, particularly at the surface of the particles. From the same sample of Porasil, columns were packed and calibrated in a gel permeation chromatograph. The availability of high molecular weight fractions of polystyrene allowed the exclusion limit of these materials to be determined for the first time. The heterogeneous pore structure did not seem to affect the effectiveness of these materials for macromolecular separation. Copyright © 1973 John Wiley & Sons, Inc.