When solutions of naphthalene in mixed alkanes or alcohols are irradiated at 315 nm or shorter wavelengths, naphthalene is destroyed by a series of reactions whose rates are greatly accelerated at high pressures. Analyses of the photoproducts recovered from diamond-anvil high pressure cells by gas chromatography-mass spectrometry demonstrate that several reactions are involved : 1) sensitized photolysis of solvent molecules to alkyl and alkoxy radicals ; 2) reduction of naphthalene to tetrahydronaphthalene and hydronaphthyl radicals ; 3) polymerization of the hydronaphthyl and alkyl radicals to dimers, trimers and higher polymers ; 4) photoaddition of solvent radicals to naphthalene ; and 5) H-D exchange between naphthalene and the solvents. The dependence of rate of disappearance of naphthalene on the excitation intensity shows that the primary photochemical step involves two-photons and triplet naphthalene intermediates that sensitize production of the free radicals which, at high pressures, are efficient consumers of unsaturated bonds. Implications of these and other phenomena described in the high pressure literature for the stabilities of unsaturated organic compounds at high pressures are discussed