An absolute value is obtained for the isotopic abundance ratio of bromine using thermal emission mass spectrometers calibrated for bias by the use of samples of known isotopic composition prepared from nearly pure separated bromine isotopes. The resulting absolute Br79/Br81 ratio is 1.02784 ±0.00190 which yields an atomic weight (C12=12) of 79.90363 ±0.00092. The indicated uncertainties are overall limits of error based on 95 percent confidence limits for the mean and allowances for the effects of known sources of possible systematic error plus a component to cover possible natural variations in isotopic composition although no provable variations were noted among the Br79/Br81ratios of 29 commercial and natural samples. Mass spectroinetric determinations of the atomic weights of bromine and silver give a combining weight ratio of AgBr/Ag= 1.740752.