The microbial degradation of hydrocarbons was studied in an artificially contaminated sandy soil, using a pilot-scale percolator system. After a short lag period, an intensive degradation occurred, which diminished in time and completely stopped in the end, despite large residual contaminations (residues of 56% diesel fuel, 20% n-hexadecane and 3.5% phenanthrene at the initial loadings of each 3000 mg/kg). The remaining pollutant content was influenced by the kind of hydrocarbon but was nearly independent of its initial loading. According to a model-aided analysis of the carbon dioxide production during remediation, the observed stagnation of degradation was caused by a limited bioavailability of the pollutants. The degradation in the soil-free aqueous phase was more extensive than in the soil, which suggests that the limited bioavailability in the soil can be attributed mainly to matrix-dependent rather than substrate-dependent influences. Generally, fine particles and organic matter are mainly responsible for the adsorption of pollutants to the soil matrix. Our sandy soil also bound hydrocarbons adsorptively although it contained neither silty material nor significant amounts of organic matter. As shown by Brunauer Emmett Teller (BET) analysis, the soil particles were covered by micropores, which enlarged the soil surface by a factor of 120 in comparison with the macroscopic surface area. The microporosity is the reason for the hydrocarbons being more strongly adsorbed to the sandy soil than expected.