The ionic transference number of semiconducting Co1-deltaO has been measured as a function of oxygen activity in the entire stability range of the phase at 1000, 1100, 1200 and 1300 degrees C, respectively, via a modified Tubandt-like electrotransport technique. The transference number falls in the range of 10(-6)-10(-4) and systematically exhibits a minimum on each transference-number isotherm. The ionic conductivity is extracted by combining with the literature data on the total conductivity of Co1-deltaO. Experimental details are given and the results are exhaustively compared with the literature data. Non-negligible interference between the flows of ionic and electronic carriers is indicated.