Lifetimes of rotational/fine-structure levels of electronically excited BH(b(3) Sigma(-)), in vibrational levels v’ 0-4, were determined from fluorescence decay waveforms with laser excitation on isolated rotational lines in the b(3) Sigma(-)-a(3)II v = 0 sequence. BH was prepared in a pulsed supersonic beam by 193 nm photolysis of diborane. The measured lifetimes were compared with values obtained from a theoretical treatment of the excited state dynamics, in which both radiative decay to the a(3)II state and nonradiative decay through the repulsive 1(3) Sigma(+) state were considered. The experimental and theoretical lifetimes agree very well. This comparison shows that the low b3 Sigma(-) rotational levels for v’ = 0-2 decay purely radiatively, with a rate decreasing as v’ increases. These rates are virtually independent of the fine-structure level. However, the lifetimes of the higher vibrational levels exhibit significant fine-structure level dependence, with the shortest values obtained for the F-2 levels. This behavior is shown to be attributable to predissociation induced by b(3) Sigma(-)similar to 1(3) Sigma(+) spin-orbit coupling.