We report the mass-selected resonance-enhanced multiphoton ionization (REMPI) spectrum of photolytically produced diatomic boron hydride over the spectral range from 368 to 372 nm. Features recorded for time-of-flight-resolved cation masses 12, 11 and 10 amu are assigned to the one-photon transition, A(1)II (v ' = 2)-X(1)Sigma (+) (v " = 0), in (BH)-B-11 and (BH)-B-10. Cation production at these wavelengths requires three photons. This signal must therefore reflect resonant one-photon absorption followed by two-photon ionization in a (1 + 2)-photon process. This work represents the first observation of the well-characterized A(1)II-X(1)Sigma (+) system in ionization spectroscopy and the first measurement of line positions in the A(1)II v = 2 state for (BH)-B-10.