Bifunctional catalysts for oxygen electrodes, active for both the oxygen reduction and evolution reactions (ORR, OER), are highly desirable for the development of electrochemical energy conversion devices. In this work, carbon-supported tantalum based catalysts were synthesized by a microemulsion procedure from tantalum ethoxide through a fast hydrolysis reaction followed by heat treatment. The nature and composition of the different tantalates and oxides (general formula NaaTabOc, with a/b = 0-1 and c/b = 2.5-3) relied on the concentration of ethanol as well as on the annealing temperature, with Na2Ta8O21 as predominant crystal phase in all cases. High annealing temperatures resulted in a certain degree of oxygen substoichiometry, which creates defects responsible of electrocatalytic activity. The electrochemical response for the oxygen reduction and the oxygen evolution reactions in alkaline electrolyte (0.1 M NaOH) was evaluated for the first time in this type of materials. The presence of Na2Ta8O21 phase over other oxides/tantalates, a high crystallinity of such Ta-phase, and the oxygen substoichiometry are the three parameters playing a relevant role to favor ORR and OER electro-activity. The best formulations were found in a trade-off situation using 3% ethanol and annealing at 900 degrees C. (C) 2019 Elsevier Ltd. All rights reserved.