Using MP2/6-311 ++G(d,p) level of theory, nine stable complexes with six-member ring structures of interactions of RCHO (R = H, F, Cl, Br, CH3, NH2) with HCHO were observed on the potential energy surface. Obtained binding energies at the CCSD(T)/6-311++G(d,p)//MP2/6-311++G(d,p) level, which include both ZPE and BSSE corrections, are in the range of 7.5-10.6 kJ.mol-1 for the above-mentioned complexes, and they are therefore quite stable. These complexes are stabilized by the blue shifting hydrogen bonds of the C-H...O and C-H...X (X = F, Cl, Br) types. By NBO analysis, it is remarkable that the contraction of the Cl-H2 and C5-H6 bonds involved in hydrogen bond and the increase in their corresponding stretching frequencies depend on both the increase in the s-character percentage of the Cl(H2) and C5(H6) atoms and the decrease in the occupation of the cr*(C1-H2) and cr*(C5-H6) orbitals, as compared to these of relevant monomers.