The reactivity of both isomers (Z and E) of pyrifenox (2',4'-dichloro-2- (3-pyridyl)acetophenone (E,Z)-O-methyloxime), adsorbed on quartz-aerosol as an inert carrier, against OH radicals from the gas-phase was investigated in a smog chamber (2400 l) at T = 299 K and 50% rel. humidity. OH-radicals were produced in the dark using hydrazine/ozone and peroxynitric acid/NO and by photolysis using methyl nitrite, hydrogen peroxide, ozone and nitrogen dioxide as OH-precursors. OH-radical concentrations (< 10⁵ < [OH]@?/cm^-3 < 8.10⁶) were determined from the simultaneous consumption of up to 6 reference compounds with well-known OH-rate constants. Using methyl nitrite, nitrogen dioxide and peroxynitric acid as OH-precursors the OH-rate constant at T = 299 K for pyrifenox was found to be identical for both isomers: (k(OH) ± σ) = (1.8 ± 0.4).10^-11 cm³.s^-1 leading to half-lives of pyrifenox in the atmosphere of t(1/2) = (0.45-0.9) days at typical mean, tropospheric OH-radical concentrations in the range of [OH]@? = (5-10).10⁵ cm^-3. Side reactions of HO₂ or other radicals perturbed the degradation behaviour of pyrifenox in the presence of hydrazine/ozone and hydrogen peroxide and ozone under irradiation and were separated from OH. The rate constants for the reaction with gaseous hydrogen peroxide and ozone were determined, found to be low and negligible with respect to the degradation of pyrifenox in the environment: (k(O₃) ± σ) = (2 ± 1).10^-19 cm³ s^-1 and (k(H₂O₂) ± σ) = (8 ± 8).10^-20 cm³ s^-1. Besides the reaction with OH-radicals a photo-isomerization Z ⇆ E was found. Within the error of experiments no irreversible loss of pyrifenox due to photolysis was found. A ketone (after reaction of OH at the CH₂-group in pyrifenox) was tentatively assigned as the only product found.