The thermochemical properties and ring strains in cyclic [n]metaphenyleneacetylenes ([n]CMPAs), butadiyne-bridged [4n] metacyclophynes (B-B[4n]MCs), and butadiyne-bridged [4 n]paracyclophynes (B-B[4n]PCs) were studied using a homodesmotic reaction scheme coupled with density functional theory (B3LYP/6-31G, mPW1PW91/6-31G, and M06-2X/6-31+G//B3LYP/6-31G). Strain energies of [n]CMPAs and B-B[4n]MCs decrease first from very high values for small rings to become zero when n becomes 6, then increase with n, and finally decrease as n becomes larger than 8. In the case of B-B[4n]PCs, strain energies decrease with increasing n. Heats of formation of [n]CMPAs, B-B[4n]MCs, and B-B[4n]PCs increase steadily with increasing numbers of phenylacetylene and 1-(buta-1,3-diynyl)benzene to reach a near-constant value per unit monomer as n increases. The geometries and (vibrational and nuclear magnetic resonance) spectroscopic properties of [n]CMPAs, B-B[4n]MCs, and B-B[4n]PCs were also studied. Geometrical parameters, Raman frequencies, and 1H NMR chemical shifts of CMPA and CMPA are found to be in good agreement with compounds for which there are experimentally available values at the B3LYP/6-31G* level of theory. In addition, electronic structure calculations were carried out for [n]CMPAs, B-B[4n]MCs, and B-B[4n]PCs. Ring diameters were also calculated for B-B[4n]PCs. © 2010 American Chemical Society.