Naphthalene quenches the excited state *UO22+ in two parallel pathways: oxidation to naphthalene radical cation, C10H8.+ (Phi = 0.3), and exciplex formation. The overall rate constant in aqueous acetonitrile containing 0.1 M H3PO4 has a value k(q) = 2.20 x 10(9) M-1 s(-1). In the presence of O-2, a portion of C10H8.+ is converted to 2-formylcinnamaldehyde and several other products, the rest reacting with UO2+ by back electron transfer.