The photochemical stability of 2,6,6-trimethyl-3-carbmethoxy-4-phenyl-5-oxo-1,4,5,6,7,8-hexahydroquinoline (HHQ) derivatives with different substituents on the phenyl ring (-Cl, -NO2, -CF3, -CH3, -OCH3) has been studied. The process of photodegradation was studied by UV spectrophotometry. The rate of photodegradation was found to depend on the type and position of the substituent in the phenyl ring. The compounds most susceptible to the damaging effect of light proved to be those containing the nitro group, in particular with the substituent in the ortho position of the aromatic ring. Derivatives with alkyl (-CH3) and halo-alkyl (-CF3) substituents showed the greatest photochemical stability The compounds with substituents in the ortho position were found to be much less photostable than the meta isomers. The quantum yield values obtained ranged from 10(-4) to 10(-3), indicating the occurrence of secondary photochemical processes initiated by the primary products of decomposition.