Ionized 1,2-dihydronaphthalene (C10H10+) and 9,10-dihydrophenanthrene (C14H12+) are homologous dihydrogenated polycyclic aromatic hydrocarbons containing adjacent sp(3) carbon sites. Tandem mass spectrometry involving kiloelectronvolt collision induced dissociation was employed to aid in the structural characterization of the products of the main dissociation channels, loss of H (and subsequent H and H-2 losses in dihydronaphthalene) and CH3. Evident from both the CID mass spectra and the imaging photoelectron-photoion coincidence (iPEPICO) breakdown curves is the fact that there are two competitive routes to the loss of H. For 1,2-dihydronaphthalene these give activation energies of 2.22 +/- 0.10 and 2.44 +/- 0.05 eV, whereas only 2.37 +/- 0.12 eV was obtained for 9,10-dihydrophenanthrene. The two parallel H-loss chaneels are believed to be the result of isomerization taking place to the methylindene ion and the 9-methylfluorene ion for 1,2-dihydronaphthalene and 9,10-dihydrophenanthren, respectively. Each newly formed isomer dissociates by H loss (one of the two competing H-loss reactions) and, of course, methyl loss. Methyl radical loss has similar kinetics for the two systems, E-0 = 2.57 +/- 0.12 eV, Delta double dagger S = 18 +/- 11 J K-1 mol(-1) for ionized dihydronaphthalene and E-0 = 2.38 +/- 0.15 eV, Delta double dagger S = -3 +/- 15 J K-1 mol(-1) for ionized dihydrophenanthrene, but as can be seen, the E-0 and Delta S-double dagger are slightly lower for the latter. The final bond rupture step in both H and CH3 loss is expected to be accompanied by a positive Delta S-double dagger, thus the low energy H loss and CH3 loss originate from the isomer ion in both cases, with the entropy of activation being dominated by the isomerization step. In contrast, sp(3)-H loss from the dihydro-PAHs differ by little in both systems (E-0 = 2.44 eV in ionized dihydronaphthalene and 2.37 eV in ionized dihydrophenanthrene and the Delta double dagger S values are 27 and 18 J K-1 mol(-1), respectively). The presence of a second sp3 carbon site has decreased the CH bond dissociation energy relative to protonated naphthalene and protonated phenanthrene, possibly to facilitate the restoration of the unaltered PAH ion. The calculated dihedral angle is -34.3 degrees in C10H10+center dot whereas C14H12+center dot has an angle of -49.6 degrees, indicating that to restore the planar nature of the molecules, which is required for all reaction channels investigated, there is more rearrangement needed for 9,10-dihydrophenanthrene. Energetics and entropic values associated with H and H-2 loss from [M - H](+) ions from ionized dihydronaphthalene were determined to be 2.72 eV, 9 +/- 17 J K-1 mol(-1), and 2.85 eV, 9 +/- 7 J K-1 mol(-1), respectively.