The morphology of soil pores is a crucial factor in the understanding of the ecology of soil microorganisms at a small scale. Xray computer tomography (CT) enables to visualize the soil pore space in three dimensions. We aimed at exploring possible changes in porosity and pore morphology during the incubation of soil to which fresh residue was added. These changes were compared to changes in porosity and pore morphology in field aggregates. A set of CT images is presented from newly formed, incubated and field aggregates from both a sandy loam and a silt loam soil. These aggregates are about 6-8 mm diameter. After incubation of the artificial aggregates, we observed the formation of micro-cracks. At the same time, the total void porosity > 27 mu m increased from 3.5% to 7%. The pore size distribution indicated that this extra porosity was present in the 27-67 mu m range. Furthermore, it was found that this new pore space was (partly) located near decomposing residue. To quantify the observed changes in pore morphology, we explored the use of the mass fractal dimension and variograms. After correction for differences in porosity, we found that incubation decreased the mass fractal dimension to the level of field aggregates in the sandy loam soil, while no differences were observed in the silt loam soil. In contrast, variogram parameters of the different treatments did not change consistently with the observations in both soils. In conclusion, the data suggest that microbial activity changes the morphology of the pore structure. However, the observed changes in the pore architecture were not sufficient to explain the large differences in aggregate stability among the analyzed samples. (c) 2005 Elsevier B.V. All rights reserved.