Various spectroscopic and photochemical properties of the acenaphthylene radical cation have been determined. Acenaphthylene cations were generated by low energy electron impact, deposited in solid argon at 12 K, and studied via Fourier transform infrared (FT-IR) and visible/UV absorption spectroscopy. In addition, the gas-phase IR spectrum of cationic acenaphthylene was obtained via multiphoton dissociation spectroscopy of the species stored in a quadrupole ion trap, using the intense and widely tunable radiation of a free electron laser. These two sets of results have been compared to the calculated (B3LYP/6-31G(d) and BP86/6-31G(d)) vibrational spectra of neutral and cationic acenaphthylene to aid in spectral band assignments. Large differences between the calculated IR intensity distributions of neutral and cationic acenaphthylene are predicted. The observed spectra are consistent with the predictions. The conversion of acenaphthylene (C12H8) into acenaphthene (C12H10) was observed upon deposition of acenaphthylene in an argon matrix with excess hydrogen atoms. The inverse conversion of acenaphthene (C12H10) to acenaphthylene (C12H8) was found to occur when the former species was exposed to UV radiation in the matrix. Calculations of the electronic excited states of the acenaphthylene cation and its hydrogenated cationic and neutral forms have been performed using time-dependent density functional theory (TDDFT), with SVWN/6-31(d,p), BLYP/6-31G(d,p), and B3LYP/6-31G(d,p) functionals/basis sets. Ten low-lying excited states were found theoretically for the cationic species. Three of these match closely with observed optical band energies. Finally, the photofragmentation pathways of the acenaphthene cation, a dihydrogenated product of acenaphthylene cation, were determined using Fourier transform ion cyclotron resonance (FT-ICR) mass spectrometry. The possible contribution of the acenaphthylene cation to the "unidentified interstellar infrared (UIR)" bands is discussed briefly.