In this study, silicon oxycarbide (SiOC) ceramic fiber mats obtained by electrospinning of two different preceramic polymers (MK and H44 resin) were evaluated in terms of their total porosity and other structural characteristics using three different characterization tools. The tensile strength and the permeability of the fiber mats were also investigated. The results indicated that the porosity could be easily calculated based on the apparent density and true density of the fiber mats obtained by gas pycnometry. A modified mercury intrusion porosimetry, in which the bulk volume of the fiber mats was calculated based on its independently measured bulk density, also allowed for an accurate evaluation of the porosity and the pore size distribution of the fiber mats. X-ray computed tomography was able to provide various structural characteristics of the 3D morphology of the fiber mats, but it was less effective in the determination of the total porosity due to resolution limits. All results showed that the MK-derived SiOC fiber mats possessed a higher porosity than the H44-derived SiOC fiber mats, resulting in a higher gas permeability. The ceramic fiber mats possessed a suitable permeability for filtration applications in harsh environments.