Publication costs assisted by the Petroleum Research Fund A new chemical actinometer especially designed for intensity measurements on high power lasers in the uv to green region is described. The actinometer consists of a closed 02-filled system containing methanolic tris(2,2'bipyridine)ruthenium(II) chloride ([Ru(bpy)3]Cl2) and tetramethylethylene (TME). The highly colored Ru(bpy)32+ absorbs the laser radiation to form a long-lived charge-transfer excited state which is efficiently quenched by dissolved O2 to form singlet oxygen (102). The 102 reacts efficiently with TME to give a nonvolatile hydroperoxide. The laser intensity is determined from the rate of 02 consumption monitored on a gas buret. The system is easy to use, has a quantum flat response over the 280-560-nm region, and is particularly suited for the ionized Ar laser lines. Detailed characterization and use of the system are presented,