The reactivity of naphthalene and pyrene radical cations and their derivatives (C10Hn+, n=6,7,8,9), C16Hn (n=9,10,11)has been studied with molecules of interstellar interest in an ion cyclotron resonance apparatus. The radical cations C10H8+ and C16H10+ are unreactive with N-2,CO,H2O and NH3. Adduct formation is the only channel for almost all reactions of C10H7+ with these molecules. The implications of these results for the stability of polycyclic aromatic hydrocarbon (PAH) cations in the interstellar medium are briefly discussed. Exploratory studies of the ion chemistry of a larger PAH, coronene, have also been done.