Using C11as tracer for the study of carbon dioxide absorption in plants, unexpected analytical difficulties were encountered. However, a satisfactory technique was developed for the rapid measurement of radioactivity in liquid samples. A degree of precision was obtained so that after the elapse of 6 half‐lives (2 hours) the sum of the activities taken from the reaction system was accurate to 1 or 2 percent. This required the determination of an accurate half‐life value for C11(20.35 min.). The experimental procedures are described.