We present laboratory studies examining the photolysis products of isovaline (Ival) and its precursor molecule, 5-ethyl-5-methylhydantoin (EM-Hyd) against continuous ultraviolet (UV), monochromatic UV, and γ-ray photons. The irradiation experiments were performed with Ival and EM-Hyd in aqueous state. In both UV and γ-ray irradiation experiments, Ival produced serine and alanine as common photolysis products. On the other hand, hydantoin and 5-methylhydantoin were detected as photolysis products of irradiated EM-Hyd. This experimental result may point out the potential importance of the prebiotic photochemistry of Ival and EM-Hyd in Solar System materials.