A new ion-pair state of Br 2, β 1 g , has been investigated by state-selective spectroscopy using the sequential two-photon transitions β 1 g ← A 3Π 1 u ← X 1Σ g +. Merged data for the three naturally occurring isotopes in the range v = 0–8, expressed in terms of the spectroscopic constants for 79Br 2 (3σ uncertainties in parentheses), are (in cm −1): T e = 49390.56(12), ω e = 151.667(42), ω e χ e = 0.4120(39), B e (e) = 0.042100(29), B e (f) = 0.042159(41), α e (e) = 1.479(57) × 10 −4, and α e (f) = 1.479(81) × 10 −4. The Ω-doubling splitting, q = B e (e) − B e (f) = −0.000059(35), is consistent with the pure precession hypothesis for heterogenous coupling between β and the E O g + state at 49 778 cm −1. Data pertaining to high vibrational levels of the A 3Π 1 u state, obtained as a byproduct of these measurements, are discussed in relation to the long-range potential of Br 2.