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 °N) and the degradation kinetics and disappearance quantum yields are reported. Fenitrothion degradation confirmed to first order kinetics (r² = 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.