Abstract. Heterogeneous catalysts, in which 'heavy' (high Z) nanoparticles are supported on 'light' (low Z) supports, are extremely important for production of bulk chemicals. The efficiency of these catalysts is closely controlled by their three dimensional (3D) structure. This paper will examine the 3D distribution of bimetallic nanoparticles embedded within a mesoporous silica support, MCM-41, using high angle annular dark field (HAADF) scanning transmission electron microscopy (STEM) tomography. This technique has shown itself to be more suitable than conventional TEM electron tomography for these specimens as it offers the advantage of a reduced damage rate and a high contrast incoherent signal. Analysis of 3D structure of these catalysts is demonstrated in both a qualitative and quantitative manner.