A total of 12 new thermochemical group additivity values (GAVs) needed to estimate the thermochernistry of large polyunsaturated hydrocarbons are derived from G2 calculations, including the nine missing hydrocarbon group values involving C-H bonds. The CBS-Q method is found to perform very poorly for the calculation of enthalpies of formation of alkynes and systems containing delocalized multiple bonds. The nonsystematic deviations between (i) CBS-Q and experimental values and (ii) CBS-Q and G2 values suggest errors in the CBS-Q extrapolation protocol rather than in the bond-additivity corrections. A few hydrocarbon groups are calculated to have surprisingly different stabilities than the others, and molecules containing these groups would be good targets for future experimental work.