An absolute abundance value has been obtained for the isotopic abundance ratio of a reference sample of gallium (Standard Reference Material 994), using thermal ionization mass spectrometry. Samples of known isotopic composition, prepared from nearly isotopically pure separated gallium isotopes, were used to calibrate the mass spectrometers. The resulting absolute 69 Ga/71Ga ratio is 1.50676 [plus or minus] 0.00039, which yields atom percents of 69Ga = 60.1079 [plus or minus] 0.0062 and 71Ga = 39.8921 [plus or minus] 0.0062. The atomic weight calculated from this isotopic composition is 69.72307 [plus or minus] 0.00013. The indicated uncertainties are overall limits of error based on two standard deviations of the mean and allowances for the effects of know sources of possible systematic error.