Ion drift in a reverse‐biasedn‐pjunction has been used to measure effective drift mobilities of Li+between 20°–125°C in Si samples containing oxygen concentrations up to 1.3×1018atoms/cm3. Previous experiments have indicated that under such conditions, the Li+and oxygen will combine to form a complex of the form LiO+. The ratio of the effective drift mobility of Li+in such samples to the known drift mobility of free Li+, when measured as a function of temperature, can be used to determine the dissociation constant of LiO+. Thus, at an oxygen concentration of 1.3×1018atoms/cm, the diffusion constant at 50°C is reduced from 1.7×10−13cm2/sec to (7.6±2.2)×10−16cm2/sec by the Li+&sngbnd;O interaction. From this and measurements at other temperatures, the dissociation constant of LiO+can be calculated, yieldingC=(5.8×1015)±30% at 50°C with an activation energy of 0.42±0.03 ev, or more generally,C=(0.5–8.5)×1022exp−[(0.42±0.03)e/kT]. At high concentrations the effective drift mobility is found to be inversely proportional to the oxygen concentration, in accordance with the model. The use of this technique for the measurement of oxygen concentrations below 1017atoms/cm3is discussed, and some examples of such measurements are given.