The mechanism of the cycloaddition reaction between singlet dichlorosilylene germylidene (Cl2SiGe:) and formaldehyde has been investigated with the CCSD(T)//MP2/6-31G* method. From the potential energy profile, it could be predicted that the reaction has one dominant reaction pathway. The reaction rule presented is that the two reactants first form a four-membered Si-heterocyclic ring germylene through the [2 + 2] cycloaddition reaction. Because of the 4p unoccupied orbital of Ge atom in the four-membered Si-heterocyclic ring germylene and the π orbital of formaldehyde forming a π→p donor–acceptor bond, the four-membered Si-heterocyclic ring germylene further combines with formaldehyde to form an intermediate. Because the Ge atom in intermediate undergoes sp3 hybridization after transition state, then, the intermediate isomerizes to a spiro-heterocyclic ring compound involving Si and Ge via a transition state. © 2012 Wiley Periodicals, Inc.