The porosity of monolithic silica columns is measured by using different analytical methods. Two sets of monoliths were prepared with a given mesopore diameter of 10 and 25 nm, respectively and with gradated macropore diameters between 1.8 and 7.5 microm. After preparing the two sets of monolithic silica columns with different macro- and mesopores the internal, external and total porosity of these columns are determined by inverse size-exclusion chromatography (ISEC) using polystyrene samples of narrow molecular size distribution and known average molecular weight. The ISEC data from the 4.6 mm analytical monolithic silica columns are used to determine the structural properties of monolithic silica capillaries (100 microm I.D.) prepared as a third set of samples. The ISEC results illustrate a multimodal mesopore structure (mesopores are pores with stagnant zones) of the monoliths. It is found by ISEC that the ratio of the different types of pores is dependent on the change in diameter of the macropores (serve as flow-through pores). The porosity data achieved from the mercury penetration measurement and nitrogen adsorption as well of scanning electron microscopy (SEM) and transmission electron microscopy (TEM) pictures are correlated with the results we calculated from the ISEC measurements. The ISEC results, namely the multimodal pore structure of the monoliths, reported in several publications, are not confirmed analyzing the pore structures of the different silica monoliths using all other analytical methods.