The degradation of the insecticide fenamiphos was undertaken by using a homogeneous photocatalysis employing iron (III) aqua complex, [Fe(H2O)5(OH)]2+. The irradiation was studied by excitation at 365 nm and the generated primary by-products were elucidated by Liquid chromatography/electrospray ionization quadrupole time-of-flight mass spectrometry in the positive ion mode, that leads to the formation of the protonated forms of the studied compounds, [M + H+]. Collision-induced dissociation tandem mass spectrometry (CID-MS/MS) of the protonated molecules was carried out, and the effect of the collision energy as well as the elemental compositions of the product ions were used in order to propose the chemical structures. The primary processes were then i) hydroxylation on the aromatic ring; ii) the loss of propene from the aminoisopropyl chain and iii) the loss of the methylene at the ethoxy moiety.