UV-Irradiation of 4-bromo-3-methyl-1-phenyl-4,5-dihydro-pyrazol-5-one (1) in the presence of various aro-matic hydrocarbons gave different types of photoproducts depending on the nature of the hydrocarbons via an elec-tron-transfer mechanism. In the presence of naphthalene or phenanthrene a photochemical homocoupling reaction of 1 occurred to form 2 or 2 and 3, respectively. UV-Irradiation of 4-bromo-3-methyl-1-phenyl-4,5-dihydro-pyrazol-5-one (1) in the presence of various aro-matic hydrocarbons gave different types of photoproducts depending on the nature of the hydrocarbons via an elec-tron-transfer mechanism. In the presence of naphthalene or phenanthrene a photochemical homocoupling reaction of 1 or 2 and 3, respectively.