For surface-treated filter media porosity properties at and just below the filter surface are essential for the particle penetration and particle clogging behaviour. In order to compare the porosities of different surface-treated filter media, a determination method, which works with a transmitted light microscope and an analysis software was developed. Using the measuring results of this method in combination with test results (clean gas concentration and particle deposition) of a VDI 3926 type 2-test equipment a linear relationship between the clean gas concentration and the surface porosities of the filter media (effective surface porosity) can be found. The effective surface porosity is determined by a pore blocking diameter d(b), below all pores remain blocked and above remain open during the stationary cycled filtration procedure. This value of db seems to be constant for different surface-treated filter media, presuming the tests have been executed at the same test equipment with the same test dust. Further a so-called pore volume equivalent can be determined by the microscopic measurement, which can be also linearly related to the deposited dust mass inside the filter media. With these two linear relationships the particle penetration and the dust holding capacity can be estimated for different surface-treated filter media. (c) 2007 Elsevier B.V. All rights reserved.