Cycloaddition reactions of the singlet CCl2 carbene with H2CO, CH3CHO and C6H5CHO and the subsequent rearrangement processes have been investigated with density functional theory. It was found that the interaction between the sigma orbital of CCl2 and the C=O pi* orbital plays an important role in the cycloaddition reactions. The addition reaction of CCl2 with H2CO proceeds very easily, which is slightly influenced by the methyl and phenyl substitutions. However, the substitutions significantly reduce the barrier to the rearrangement reactions. The interaction between solvent and solute has a little influence on the structures of the stationary points, but has a noticeable influence on the barrier heights of the rearrangement reactions. (C) 2003 Elsevier B.V. All rights reserved.