Experimental retention data with several membrane filters show that the filtration efficiency decreases considerably over time, which appears to be inconsistent with a size exclusion retention mechanism. Several model descriptions of the porous structure are considered to describe this behavior. Simulation results with a sieving mechanism are compared to experimental data and the influence of input parameters are investigated. Fairly simple models may be used to qualitatively describe the retention behavior. More complex models are necessary to allow for a more accurate description of the physical geometry of the structure and a better quantitative representation of the experimental data.