HMO calculation estimated a tendency of photochemical reaction of quinolinecarbo-nitriles and isoquinolinecarbonitriles in presence of organic acid. The tendency was proved by irradiation on these compounds in the presence of propionic acid and by identification of the reaction products. 2-Ethylquinoline, 2, 4-diethylquinoline, 2-phenylquinoline and 4-ethyl-2-quinolinecarbonitrile were given by the reaction of 2-quinolinecarbonitrile. 3-Quinolinecarbonitrile gave 2-ethyl-3-quinolinecarbonitrile, 4-ethyl-3-quinolinecarbonitrile, 3-cyano-4-ethyl-1, 4-dihydroquinoline and 2, 4-diethyl-3-quinolinecarbonitrile and 4- quinolinecarbonitrile gave 4-ethylquinoline, 2, 4-diethylquinoline and 2-ethyl-4-quinolinecarbonitrile as the reaction products. In isoquinoline series, 1-isoquinolinecarbonitrile gave 1-ethylisoquinoline, 1-phenylisoquinoline, 1, 4-diethylisoquinoline and 4-ethyl-1-isoquinolinecarbonitrile and 4-isoquinolinecarbonitrile gave 1-ethyl- 4-isoquinolinecarbonitrile and 4-cyano-l-ethyl-1, 2-dihydroisoquinoline.