The inclusion complexes of 4-X-benzaldehyde guests (X = F, Cl, Br) in alpha-, beta-and gamma-cyclodextrins (alpha CD, beta CD and gamma CD) were prepared and characterized by means of thermogravimetry, differential scanning calorimetry (DSC) and vibrational spectroscopy. The stoichiometry of the inclusion complexes ranges from 2: 1 to 1: 2, depending on the cavity size of the host. Sensitive vibrational modes such as C=O and ring CH stretching modes were used to monitor the effects of the inclusion process and to assess the preferred inclusion geometry for each host-guest pair. On the whole, the spectral observations suggest that the small aCD cavity imposes important structural restrictions on the guest molecule, while the larger gamma CD cavity allows ‘liquid-type’ intermolecular contacts.