The concentration of35SO2in air can be measured by reacting with Pbo2to form Pb35SO4. A method was developed which is suitable for rapid, high-protection radioassaying of35SO42-on the surface of Pbo2. The surface35SO42-was extracted with a quaternary alkylammonium hydroxide in toluene and counted in an organic liquid scintillator solution with an efficiency of approximately 94%, which was constant for 30 days when corrected for natural decay. More than 99.5% of the surface sulfate was extracted in less than 5 minutes for quaternary alkylammonium concentrations 600 times in excess. This technique was shown superior to gas-flow proportional counting of the PbO2surface.