The highly congested spectra of the 0-0, 1-0, 0-1, and 1-1 bands of the B 2 Σ + <-X 2 Σ + system of YbBr have been analyzed for the 1 7 4 Yb 7 9 Br, 1 7 4 Yb 8 1 Br, 1 7 2 Yb 7 9 Br, and 1 7 2 Yb 8 1 Br isotopomers. A multi-isotopomer, multi-band fit of all data was employed in the determination of an isotopically consistent set of equilibrium parameters. From the fitted B e values, estimates of the equilibrium bond lengths for the B and X states of YbBr were calculated as 2.58275(1) and 2.645386(4)A, respectively. Differences in the electronic term values between the 1 7 2 YbBr and 1 7 4 YbBr isotopomers have been interpreted in terms of the Field Shift Effect. This effect, well known in atomic spectroscopy, is the result of a small difference in the nuclear charge distribution between the 1 7 2 Yb and 1 7 4 Yb nuclei, and has been observed for very few molecules.