The determination of quantum yields for the photolysis of Fe(III)-hydroxo complexes is important for the quantitative investigation of hydroxyl radical (OH) production, not only in a natural water body, but also in the photo-Fenton process. A novel kinetic method, using a OH probe compound, was established for the determination of the quantum yields in this study. The method was based on measuring the pseudo-first-order rate constant of the photodecomposition of dimethylsulfoxide (DMSO) in which DMSO and its primary products scavenged the OH at an identical rate. The preliminary experiments for the photodecomposition kinetics supported the suitability of DMSOs as a probe compound for determining quantum yields. The individual quantum yields for the photolysis of the monomeric Fe(III) complexes, in the wavelength range 240–380 nm, were determined by the photodecomposition kinetics of the hydroxyl radical (OH) probe compound (DMSO). The determined values of the individual quantum yields were 0.046 ± 0.00052 for Fe3+ (H2O)6 (hexaaquo ion) and 0.69 ± 0.025 for Fe(OH)2+(H2O)5 (hydroxypentaaquo ion) at 254 nm, and showed decreasing values with increasing wavelength, in the ranges of 240–380 nm. The quantum yields between 240 and 280 nm were newly reported in this study, and the values obtained between 280 and 380 nm were in good agreement with the literature values.