This paper presents first experiments of laboratory investigations of the photodegradation by direct photolysis (λ > 290 am) of cholorotahlonil, dichlobenil, chloroxynil, bromoxynil, and ioxynil in aqueous, pH-buffered, and organic solutions and the calculation of the quantum yields. The photolysis of chlorothalonil in water is low, with a corresponding low quantum yield (Φ = 0.0001). Dichlobenil is photostable under the laboratory conditions used. The photoreactivity of bromoxynil and ioxynil was found to be comparable in aqueous solutions and about three times lower with respect to chloroxynil. The quantum yields obtained in water of chloroxynil, bromoxynil, and ioxynil are Φ = 0.0060, 0.0093, and 0.0024, respectively. Half-lives of the pesticides in the environment with respect to direct irradiation are estimated using UV spectra and quantum yields as input variables obtained in the laboratory.