Density-functional theory (B3LYP/6-31G(d,p)) and the ONIOM (Our-own-N-layer Integrated molecular OrbitalCmolecular Mechanics) approach utilizing two-layer ONIOM schemes (B3LYP/6-31G(d,p):UFF) have been employed to investigate structures of Na-exchanged zeolite-encapsulated formaldehyde (HCHO@Na-zeolite) and their interactions with propylene. The carbonyl-ene reaction of propylene and formaldehyde was studied on three model systems: (1) formaldehyde in Na-exchanged zeolite: HCHO@Na-zeolite/CH 3 CHaCH 2 ; (2) naked Na(I) as catalyst: Na(I)/HCHO/CH 3 CHaCH 2 ; (3) a bare model where only the reactants are present: HCHO/CH 3 CHaCH 2 . It is found that inclusion of the extended zeolite framework has an effect on the structure and energetics of the adsorption complexes and leads to a lower energy barrier (25.1 kcal/mol) of the reaction as compared to the bare model system (34.4 kcal/mol). If the naked Na(I) interacts with the HCHO/CH 3 CHaCH 2 complex the energy barrier of the system is even lower than HCHO@Na-zeolite/CH 3 CHaCH 2 , due to the large electrostatic field generated by the naked Na(I) cation (17.5 kcal/mol). The carbonyl-ene reaction of propylene using HCHO@Na-faujasite takes place in a single concerted reaction step.