The photoprocesses of berberine, palmatine, coralyne, sanguinarine, flavopereirine and ellipticine were studied in several solvents. The quantum yields ΦΔ of singlet molecular oxygen formation of berberine, palmatine and sanguinarine are moderate in dichloromethane (0.2–0.6) and much smaller in acetonitrile or trifluoroethanol. For the other alkaloids examined, ΦΔ is rather independent of solvent polarity. The direct and ketone-sensitized photolysis, using steady-state irradiation at 313 nm or 248/308 nm laser pulses, was studied by absorption and fluorescence spectroscopy. Thereby, radicals were observed yielding eventually dihydro derivatives as major products, which are thermally back-converted on admission of oxygen. The quantum yield of conversion of alkaloids to dihydroalkaloids is enhanced in the presence of triethylamine. The reaction in the presence of ketones and electron or H-atom donors has a quantum yield of close to unity.