The MP2/6-311++G(d,p) calculations on H2CO...(CIF)(n) complexes (n up to 6) have been performed and the halogen bonding cooperative effect has been analyzed. The `atoms in molecules' (AIM) theory was applied to study halogen bonding interaction. For the systems analyzed, the strength of (ClO)-O-... interaction increases if the number of CIF molecules increases. For this interaction, oxygen acts as the Lewis base while Cl atom as the Lewis acid, that may be explained as an effect of the anisotropy of the electron charge distribution for chlorine atom. (c) 2006 Elsevier B.V. All rights reserved.