We report on an efficient supersonic chemical oxygen-iodine laser, energized by a jet-type singlet oxygen generator, operating without primary buffer gas and applying simple nozzle geometry and transonic mixing of iodine and oxygen. Output power of 177 W with chemical efficiency of 17&percent; was obtained in a 5 cm gain length forCl2flow rate of 11 mmol/s. The power is almost unaffected by water vapor in the medium. ©1997 American Institute of Physics.