This paper describes a three-pronged study of microwave (MW) activated B2H6/Ar/H-2 plasmas as a precursor to diagnosis of the B2H6/CH4/Ar/H-2 plasmas used for the chemical vapor deposition of B-doped diamond. Absolute column densities of B atoms and BH radicals have been determined by cavity ring-down spectroscopy as a function of height (z) above it molybdenum substrate and of the plasma process conditions (B2H6 and Ar partial pressures, total pressure, and Supplied MW power). Optical emission spectroscopy has been used to explore variations in the relative densities of electronically excited BH, H, and H-2 species as a function of the same process conditions and of time after introducing B2H6 into a pre-existing Ar/H-2 plasma. The experimental measurements are complemented by extensive 2-D(r, z) modeling of the plasma chemistry, which results in refinements to the existing B/H chemistry and thermochemistry and demonstrates the potentially substantial loss of gas-phase BHx species through reaction with trace quantities of air/O-2 in the process gas mixture and heterogeneous processes occurring at the reactor wall.