The potential energy surface (PES) of the dimers of formaldehyde derivatives CH(O)Y (Y = H, CH3, F, Cl, Br, I) has been investigated by means of quantum chemical calculations at the MP2/6-311++G** level. Several minima have been found on the PES characterized by various combinations of C-(HX)-X-... (X = O, halogen) contacts. The computed dimerization energies revealed the importance of dispersion forces in the formation of [CH(O)Y](2) dimers, while only a lesser role of the intermolecular (HX)-X-... interactions. The most characteristic geometrical properties of the dimers are the (HX)-X-... distances being near the sum of the van der Waals radii of the contacting atoms, the lengthening of the contacting C-X bonds, and the general shortening of the C-H bonds by 0.001-0.004 Angstrom with respect to the monomers. The latter bond shortening is responsible for the characteristic blue-shift of the CH stretching frequencies in the dimers. A natural bond orbital (NBO) analysis revealed a slight decrease in the population of the contacting sigma*(CH) orbitals and alterations in the intramolecular charge-transfer effects as the primary reason of the C-H contraction.