The potential energy curve for the (B 0^{+}\left({ }^{3} \Pi\right)) state in (\mathrm{BrCl}) exhibits a barrier which has been attributed to the effects of an avoided crossing with another (0^{+})electronic state that correlates with two ground state atoms. Existing data for (v) levels (2-8) in ({ }^{79} \mathrm{Br}^{35} \mathrm{Cl}) and ({ }^{8 /} \mathrm{Br}{ }^{35} \mathrm{Cl}) are used to obtain closed-form representations of the (B)-state potential that are reliable to (\sim 0.01 \mathrm{~cm}^{-1}). When the data for both isotopomers are fitted simultaneously, the variance rises by a factor of 2. providing an estimate of the extent of Born-Oppenheimer breakdown in this state. From among several satisfactory closed-form potentials, there are some which approximately reproduce the observed breaking off of the high- (J) lines in the (0-8) band of the (E \rightarrow B) transition near (3290 \dot{A}). This tunneling resonance spectrum is modeled using bound-free spectral calculations, from which the (B)-state barrier is located (18520 \pm 10 \mathrm{~cm}^{-1}) above the minimum of the ground state and (275 \mathrm{~cm}^{-1}) above the first dissociation limit. 1995 Academic Press. Inc.