Small gold (5 nm) and zirconia (2-3 nm) particles inside the mesopores of SBA-15 were visualized with bright-field electron tomography. Due to the ordered nature of the mesopores and the related diffraction contrast present in the tilt series, the 3D-reconstruction is difficult to interpret. Unequivocal information has been obtained, however, on the presence of the particles in certain pores, while others pores are empty. This nonuniform distribution of particles over the mesopores explains the two-step desorption isotherm in nitrogen physisorption experiments.