The chemical reaction between HCHO and CCl2 was studied using a high level of theory. Geometry optimizations were performed at the complete active space self-consistent field (CASSCF) level with the 6-31G* basis set, and single point calculations were performed using the coupled-cluster with single, double, and perturbative inclusion of triple excitation [CCSD(T)] method. Additivity approximation of the correlation energy was used, so we have obtained an effective CCSD(T)/6-311G(2d1f,2p) reaction and activation energies. Our results show that the ylide exist as astable species, having a transient character. Its lifetime in relation to ring closure was estimated to be 1 ms at 298.15 K, and it can react backward forming the HCHO and CCl2, fragments. The cycloaddition reaction is highly competitive with ylide formation, and that is the dominant process. The reaction rate of cycloaddition is about two and a half times higher than the ylide formation at 298.15 K.