Two temperature-sensitive model photochemical reactions, the Norrish Type II reaction and photochemical nucleophilic aromatic substitution on 4-nitroanisole by the hydroxide ion, carried out in high-temperature water (100–200 °C) in a pressurized vessel under microwave heating, are reported. The observed chemoselectivity and the ability to increase the solubility of hydrophobic organic compounds in this environmentally friendly solvent are promising results for prospective green (photo)chemical applications.