Aqueous solutions of fenitrothion and methyl parathion were photochemically degraded in an Atlas Suntest solar simulator (500 W m(-2)) as well as under ambient sunlight at Lancaster University (June & August 2007, 54 degrees N) and the degradation kinetics and disappearance quantum yields are reported. Fenitrothion degradation confirmed to first order kinetics (r(2) = 0.90-0.99) with a half-life range of 4.9 h-5.3 h, shorter than previously reported studies. In contrast, methyl parathion did not show significant degradation over the duration of these experiments. Light irradiances were monitored with a spectroradiometer during the Suntest simulator and outdoor experiments. The filtered-xenon arc lamp of the Suntest yielded spectral irradiances comparable to natural sunlight in the UVB and UVA wavelength range (280-400 nm), but with higher irradiances in the visible region (400-750 nm). Nonetheless, as both compounds have light absorption spectra at wavelengths < 400 nm, then the half-lives and disappearance quantum yields were similar between the Suntest and natural sunlight, and demonstrated that the Suntest is suitable for environmentally-relevant photochemical degradation experiments.