Unimolecular fragmentations of the anthracene and phenanthrene radical cations C14H10.+ were studied by time-resolved photoionization in the vacuum UV, RRKM/QET calculations, and MS/MS with electron ionization. The primary reactions observed are parallel H-., H-2, C2H2, C3H3., and C4H2 losses, as well as two consecutive H-. losses from C14H9+ and C12H8.+, respectively. appearance energies were determined for the microsecond and millisecond time ranges. Activation parameters were deduced for the reactions. The H-. and C?H? loss reactions are characterized by loose transition states. The following heats of formation were deduced : Delta H(fO)degrees (C14H9+, anthracenyl) = 282.0 +/- 3.0 kcal/mol, Delta H(fO)degrees(C14H9+, phenanthrenyl) = 276.4 +/- 3.0 kcal/mol and Delta H(fO)degrees(C12H8.+, biphenylene) less than or equal to 281.7 +/- 3.0 kcal/mol. The C-H bond energies in the anthracene and phenanthrene radical cations are 4.38 +/- 0.08 and 3.92 +/- 0.10 eV, respectively. The role of isomerization of the parent radical cations prior to dissociation will be discussed.