Phenylacetylene scaffolds are known to be an important component in electronic devices, but their thermochemistry has not been studied so far. In this paper, we systematically calculate the strain energies (SE) and the standard enthalpies of formation (Delta(f) H-298 degrees) of sixteen linear polyynes-substituted benzene (PSB) and seven cyclic [n]orthophenyleneacetylenes ([n]COPAs) (n = 2-8). The geometries and vibrational frequencies of PSB were calculated at B3LYP/6-31 + G(d,p), mPW1PW91/6-31 + G(d,p), and M06-2X/6-31 + G(d,p) levels of theory and for [n]COPAs, mPW1PW91/6-31G(d) was used. Isodesmic and homodesmotic reactions approach were used to calculate SE and Delta(f) H-298 degrees. The results show that Delta(f) H-298 degrees of PSB increases steadily with the increasing the numbers of -C-=-C- linkage, and endothermic Delta(f) H-298 degrees of these molecules are indicative of an acetylene substituent strain. The SE per-unit monomer (SE/n), and Delta(f) H-298 degrees per-unit monomer (Delta(f) H-298 degrees/n) of [n]COPAs were calculated and results were compared with its meta and para counterpart. In addition, electronic properties such as HOMO, LUMO and HOMO-LUMO energy gap were calculated.