The energetics of acetylene loss from anthracene and phenanthrene radical cations and the relative stabilities of the six possible C12H8.+ fragments have been investigated using the hybrid density functional methods B3LYP and B3PW91. Isodesmic reactions involving the well-known fragmentation of the benzene radical cation were employed to derive the dissociation energies for the fragmentations of naphthalene, anthracene, and phenanthrene radical cations. CCSD(T) calculations were also carried out for the acetylene loss from benzene and naphthalene radical cations. The B3LYP and B3PW91 methods appear to bracket the CCSD(T) result. The result for the naphthalene radical cation agrees with the previous suggestion that the benzocyclobutadiene radical cation is the most plausible product of acetylene loss (Chem. Phys. 1995, 191, 165). Computational results for anthracene and phenanthrene will be discussed in the light of experimental data. Acetylene losses from anthracene and phenanthrene radical cations will be shown to form the same fragment, biphenylene(.+), which indicates that the isomerization barrier between the two isomers is lower than the dissociation limits. Scaled B3LYP/cc-pVDZ vibrational frequencies for naphthalene, anthracene, and phenanthrene radical cations are compared with available experimental data. Frequencies at the same level of theory are provided for C4H4.+, C8H6.+, and C(12)H8(.+) isomers as well as for the benzene radical cation and should facilitate further experimental work.