The effect of O2 on the reduction of sodium pertechnetate by stannous chloride was investigated using a paper electrophoresis method. O2 interfered with the reduction process by oxidizing stannous ion, Sn(II), to stannic, Sn(IV), thereby leaving less Sn(II) available for the reduction of pertechnetate ion. In a solution of 1.66 .times. 10-3 M TcO4- saturated with O2, a 3.4 mol ratio of Sn(II) to TcO4- was required for complete reduction of TcO4-, whereas in a N2 atmosphere, a 2.4 mol ratio was required for the same purpose. The O2 interference increased as the concentration of TcO4- decreases. In a N2 atmosphere, 1.40 and 10.05 mol ratios of Sn(II) to TcO4- were required for the complete reduction of 1.66 .times. 10-2 M and 1.66 .times. 10-5 M TcO4-, respectively. Once the reduction is accomplished, it was not easily reversible. Bubbling with O2 for 1 h had no effect on the quantity of reduced Tc.