Gas-phase DeltaH(f298)(0) of CH=CCl, CCl=CCl, CH=C., and CCl=C. are computed by QCISD(T)/6-31+G(d'), CBS-Q, CCSD(T)/6-311++G(3df,2p), and G3 methods based on B3LYP/6-311+G(d,p) and MP2(Full)/6-311+G(2d,2p) geometries, CBS-APNO is also used for CH=C.. B3LYP and MP2(Full) structures result in similar energies for C2H2, C2HCl and C2Cl2. DeltaH(f298)(0) of C2H. from B3LYP geometry is close to CBS-APNO but 2 kcal/mol below MP2(Full); the CBS-APNO result is recommended. MP2(Full) structure results in lower energies for C2Cl., MP2(FC) geometries give results within 0.05 kcal/mol of MP2(Full). Recommended DeltaH(f 298)(0) are 54.54 +/- 0.17, 53.80(+2/-5), 53.89 +/- 4.78, 135.89 +/- 1.81, 135.66 +/- 6.24 kcal/mol for C2H2, C2HCl, C2Cl2, C2H., C2Cl., respectively. S-298(0) and Cp-0 (T) are calculated. (C) 2002 Elsevier Science B.V. All rights reserved.