The structure and stability of (CoO)(x) clusters in siliceous ZSM5 (silicalite) and on SiO2 have been investigated using atomistic simulation techniques. The deconvolution of the energy into distortion and interaction contributions revealed that the improved stability of the supported clusters is due to the energy gained from the cluster-host interaction. (CoO)(x) clusters in ZSM5 of between 4 and 6 Co atoms showed an enhanced stability, while clusters larger than 18 Co atoms were sterically limited inside the zeolite lattice.