B3LYP/LANL2DZ and B3LYP/6-31G(d)-restricted and -unrestricted calculations are employed to calculate energies and adsorption forms of formaldehyde adsorbed on planar and on tetrahedral Pd-4 clusters and on a Pd4 cluster supported on Al10O15. Formaldehyde adsorbs on planar Pd-4 in the eta(2)(C,O)-di-sigma adsorption mode, while on tetrahedral Pd-4, it adsorbs in the eta(2)(C,O)-pi adsorption mode. The adsorption energy on planar Pd-4 is -21.4 kcal center dot mol(-1), whereas for the tetrahedral Pd-4 Cluster, the adsorption energy is -13.2 kcal center dot mol(-1). The latter value is close to experimental findings (-12 to -14 kcal center dot mol(-1)). Adsorption of formaldehyde on Pd-4 supported on an Al10O15 cluster leads essentially to the same result as that found for adsorption on the tetrahedral Pd-4 cluster. Charge density analysis for the interaction between formaldehyde and the Pd-4 clusters indicates strong backdonation in the eta(2) adsorption mode, leading to positive charge on the Pd-4 cluster. NBO analysis shows that the highly coordinated octahedral aluminum atoms of Al10O15 donate electron density to the supported Pd-4 cluster, while tetrahedral aluminum atoms with lower coordination number have acidic nature and therefore act as electron acceptors.