Abstract A novel type of photochemical reaction between 9,10-phenanthraquinone and glyoxal was discovered and extended to several glyoxals, i,e., methyl-, ethyl-, n-propyl-, i-propyl-, t-butyl-, cyclohexyl-, and phenyl-glyoxals. In these reactions, monoesters of 9,10-dihydroxyphenanthrene could be obtained accompanying with the vigorous evolution of carbon monoxide. The dynamic aspect of these photochemical reactions was investigated using CIDNP technique, and the reverse hydrogen abstraction reaction between two radical species which were resulted from glyoxal and photoexcited 9,10-phenanthraquinone was found to occur.