Two simple and reliable methods for constructing highly stable very uniform standard reference targets for nuclear microanalysis of oxygen-16 have been devised, based on the use of electrochemical or thermal oxidation procedures of tantalum. The electrochemical method consists in anodizing high purity tantalum at constant current density up to a well defined voltage in a well chosen aqueous electrolyte. Conditions were found in which the electronic current through the oxide film formed is less than 1%; a precise coulometric measurement gives the total amount of oxygen atoms in the film formed with an overall estimated absolute precision of ±3%, and a relative reproducibility better than ±1%. A simplified method is described, in which the only measurement of the final voltage reaached allows one to know to within ±3% the oxygen contents of the anodic oxides; this is based on the known voltage-thickness (in atoms/cm 2) relationship given in the paper, as determined once for all using the above method. The thermal oxidation method consists in heating thin tantalum films deposited on silicon for some hours at around 500°C in oxygen. The fully oxidized film, shown to be pure Ta 2O 5, is then calibrated by direct weighing or using Rutherford backscattering. The two types of standard give fully consistent results. Usual oxide thicknesses range from 100 to 800×10 15 atom/cm 2 (i.e. around 200–1600 Å). Error estimations and long term stability are discussed.