Tandem mass spectrometry was used to explore the trends in the unimolecular fragmentation of the ionized hydroxy-substituted polycyclic aromatic hydrocarbons 1 -naphthol, 9-hydroxyphe-nanthrene, and 1-hydroxypyrene. The main dissociation reactions across all ring systems were CO- and HCO-losses, with ionized 1-naphthol also losing H atoms. Both ionized 1 -naphthol and 9-hydroxyphenanthrene displayed the sequential loss of C2H2 and C4H2 from the [M-HCO](+) ions, reminiscent of unsubstituted PAH ions. CO -loss is slightly favored for 1-naphthol and 9-hydroxyphenanthrene, at low collision energy, but less so for 1-hydroxypyrene. Reaction mechanisms for HCO- and CO-losses from 1-hydroxypyrene were derived from CCSD/6-31G(d)//B3LYP/6-31G(d) calculations. The CO -loss mechanism is dominated by two transition states: TS -A governing a 1,3-H shift in the molecular ion and TS -C which governs a ring-closing step to form a five -member ring in the product ion. HCO-loss occurs over a much flatter potential energy surface with the intermediate being the product ion bound to the carbon atom of HCO. Imaging photoelectron photoion coincidence spectroscopy of 1hydroxypyrene yielded threshold photon -energy resolved breakdown curves and time-of-flight distributions that were modeled with RRKM theory to give 0 K reaction energies for HCO- and CO -losses of 3.92 +/- 0.05 and 2.91 +/- 0.05 eV, respectively. The entropies of activation for the two channels were very different, 14 and 95 JK(-1) mol(-1), respectively, a result consistent with the calculated mechanisms. The threshold photoelectron spectrum yielded an IE value of 7.14 +/- 0.01 eV for 1-hydroxypyrene.