A technique for measuring the zeta-potential of a gas-liquid interface that overcomes the valid criticisms of previous attempts is proposed and demonstrated for the case of an air bubble in deionized water. The zeta-potential is found to be -65 mV with a reliability exceeding any previously reported results. The apparatus used is a horizontal tube that is rapidly rotating about its axis. The tube contains the liquid and small gas bubbles. An electric field imposed along the tube axis causes the bubble to move and the zeta-potential is related to the electrophoretic mobility.