The keto-enol isomerization of acetaldehyde inside HZSM5 has been studied using the B3LYP density functional approach and different cluster models (T3, T5, and T63). For the largest cluster, which contains up to 63 tetrahedra, we have used the ONIOM2(B3LYP:MNDO) and ONIOM2(B3LYP:AM1) approaches. In all cases, calculations show that the zeolite produces an important catalytic effect on the enolization reaction and that the adsorption of acetaldehyde to the zeolite corresponds to a neutral H-bond complex. Enlarging the size of the cluster to T63 with ONIOM2 does not modify the nature of the interaction between the keto and enol forms of acetaldehyde with the zeolite. However, the embedding effects destabilize the keto intermediate and stabilizes the enol one, which results in a considerable decrease of the reaction energy. The ONIOM procedure allows us to naturally introduce the limited flexibility of the zeolite at a reasonable computational cost without imposing artificial constraints.