Transition structures and energy barriers of the concerted prototypical cycloaddition reaction of 1,3-heterocumulenes (S=C=S, S=C=NR, RN=C=NR, and heteroanalogs) to acetylene resulting in nucleophilic carbenes were calculated by G2(MP2) and CBS-Q ab initio quantum chemical and by density functional theory (DFT) methods. According to the calculations the activation energies (activation enthalpies) of the homoheteroatomic cumulenes decrease in the order O > S > Se and NH > PH and the reaction energies in the order O > S approximately Se and PH > NH. The reaction of carbon disulfide and acetylene has a lower reaction barrier than that of carbodiimide and acetylene although the first reaction is less exothermic than the second one. The stronger cyclic stabilization of the 1, 3-dithiol-2-ylidene in the transition state is discussed in terms of deformation and stabilization energies and of bond indices. The known reactions of carbon disulfide with ring-strained cycloheptynes were examined by DFT and by DFT:PM3 two-layered hybrid ONIOM methods. In agreement with qualitative experimental findings the activation energy increases and the reaction energy decreases in the sequence S, SO(2), and SiMe(2) if CH(2) in the 5-position of 3,3,7, 7-tetramethyl-1-cycloheptyne is replaced by a heteroatom or heteroatomic group, respectively. The results of these calculations were corroborated by experimental studies with carbon diselenide and isothiocyanates as 1,3-heterocumulenes. The cycloaddition of carbon diselenide to cyclooctyne proceeded faster than with carbon disulfide, the main product being the 1,3-diselenol-2-selone. Under more drastic conditions it was possible to add methyl and phenyl isothiocyanate, respectively, to 3,3,6, 6-tetramethyl-1-thia-4-cycloheptyne. The products are 1:3 adducts (cycloalkyne:isothiocyanate) whose formation is explained by a trapping reaction of the first formed 1,3-thiazol-2-ylidenes.