The atomic weight of a reference sample of silver has been determined by mass spectrometry, with an uncertainty of one part in10[to the 6th], using a single filament silica gel procedure. Accurately known quantities of chemically pure 107Ag and 109 Ag were mixed to produce standards of know isotopic compositions for calibration of the mass spectrometer. The absolute isotopic ratio of the reference sample of silver is 107Ag/109Ag = 1.07628 [plus or minus] 0.0022 yielding an atomic weight of 107.86815 [plus or minus] 0.00011. The indicated uncertainties represent an overall limit of error at the 95 percent confidence level which is the sum of the uncertainty components for the ratio determined and the components covering effects of known sources of possible systematic error.