Abstract : A simple analytical procedure was developed to measure with high accuracy americium isotope ratios and concentration. The method was tested through the participation in a round robin test organized by the Analytical Methods Committee of the French Atomic Energy Commission. The measurements were performed by Thermal Ionization Mass Spectrometry using the total evaporation method, which is a reference technique for actinides isotope determination. Expanded uncertainties were estimated at 0.1 percent and 0.8 percent for the 241Am-243Am isotope ratio and the Am concentration, respectively. Compared to the assigned value, a bias below 0.0001 percent was achieved for the 241Am-243Am isotope ratio and below 0.02 percent was found for the americium concentration.