A technique previously used to determine the concentration of O(3P) in the stratosphere is employed to determine the absolute concentration of ground state hydroxyl radicals between 29 and 43 kilometers in the earth's atmosphere. The results from two flights of the instrument, which detects OH by molecular resonance fluorescence while descending through the stratosphere suspended from a stabilized parachute platform, are reported. Both experiments were launched on a balloon from Palestine, Texas, 32¿N around noon and dropped the same afternoon near the stratopause at a solar zenith angle of 80¿.

From the first flight on 18 July 1975 the OH concentration is reported to be 2.0+1.5-1.0¿107 cm-3 at 40 kilometers and from the second on 12 January 1976 the density is found to decrease from 2.8¿107 cm-3 at 43 kilometers to 4.5¿106 cm-3 at 30 kilometers with an experimental uncertainty of ¿35 percent.