The accepted weight of thallium has remained at a value of 204.37 [plus or minus] 0.03 since 1962. At this level of uncertainty, however, the atomic weight becomes a limiting factor to high accuracy analysis. The new mass spectrometric determination of the atomic weight of thallium has been completed. A high precision assay technique was developed so that accurately known quanties of the 203 Tl and 205 Tl separated isotopes could b mixed to produce standard for calibration of the mass spectrometer. This assay technique involved the gravimetric determination of 99.3 percent of the thallium as Tl2CrO4. The soluble thallium was then aliquoted and determined by isotope dilution mass spectrometry. Before making up the final solutions from which the assay and calibration samples would be withdrawn, the separated isotopes were purified by solvent extraction and electrodeposition. A tungsten filament surface ionization technique was developed for the determination of precise isotopic abundance measurements for thallium. This technique allowed isotopic analysis of the separated isotopes, calibration standards, and a natural thallium reference standard with precisions of better than 0.1 percent. The 205Tl/203Tl absolute abundance ratio of the reference sample was found to be 2.38714 [plus or minus]0.00101, yielding an atomic weight of 204.38333 plus 0/00018