Using selectively detected laser-induced fluorescence, the rotational state distribution of the BaI product has been measured for the beam-gas reaction Ba + HI → BaI(ν = 8) + H. Owing to the highly constrained kinematics for this system, these measurements can be used to derive the reaction probability as a function of the impact parameter for this channel, called the "specific" opacity function, once the reaction probability as a function of velocity has been determined. Unfortunately, lack of knowledge of the exoergicity and the height of any energy barrier prevents a conclusive determination of the specific opacity function for this reaction. Instead, various approximate opacity functions are estimated based on different models of the velocity dependence of the reaction channel studied. If the reaction probability is the same for all relative collision velocities, then the BaI (ν = 8) specific opacity function peaks strongly near 2.6 Å with a full width at half-maximum of 1.0 Å. However, the possible presence of a small energy barrier in the entrance channel causes a cutoff in the relative collision velocity distribution, and this type of velocity dependence would significantly affect the shape of the specific opacity function. © 1986 American Institute of Physics.