Ultrafast relaxation dynamics for photoexcited PAH cations isolated in boric acid glass have been studied using femtosecond and picosecond transient grating spectroscopy. With the exception of perylene(+), the recovery kinetics for the ground doublet (D-0) states of these radical cations are biexponential, containing fast (<200 fs) and slow (3-20 ps) components. No temperature dependence or isotope effect was observed for the fast component, whereas the slow component exhibits both the H/D isotope effect (1.1-1.3) and a strong temperature dependence (15 to 300 K). We suggest that the fast component is due to internal D-n to D-0 conversion and that the slow component is due to vibrational energy transfer (VET) from a hot D-0 state to the glass matrix. The observed rapid, efficient deactivation of the photoexcited PAH cations accounts for their remarkable photostability and has important implications for astrochemistry because these cations are the leading candidates for the species responsible for the diffuse interstellar bands (DIB) observed throughout the galaxy.