We report a combined experimental/modeling study of microwave activated dilute N-2/H-2 and NH3/H-2 plasmas as a precursor to diagnosis of the CH4/N-2/H-2 plasmas used for the chemical vapor deposition (CVD) of N-doped diamond. Absolute column densities of H(n = 2) atoms and NH(X-3 Sigma(-), v = 0) radicals have been determined by cavity ring down spectroscopy, as a function of height (z) above a molybdenum substrate and of the plasma process conditions, i.e., total gas pressure p, input power P, and the nitrogen/hydrogen atom ratio in the source gas. Optical emission spectroscopy has been used to investigate variations in the relative number densities of H(n = 3) atoms, NH(A(3)Pi) radicals, and N-2(C-3 Pi(u)) molecules as functions of the same process conditions. These experimental data are complemented by 2-D (r, z) coupled kinetic and transport modeling for the same process conditions, which consider variations in both the overall chemistry and plasma parameters, including the electron (T-e) and gas (T) temperatures, the electron density (n(e)), and the plasma power density (Q). Comparisons between experiment and theory allow refinement of prior understanding of N/H plasma-chemical reactivity, and its variation with process conditions and with location within the CVD reactor, and serve to highlight the essential role of metastable N-2(A(3)Sigma(+)(u)) molecules (formed by electron impact excitation) and their hitherto underappreciated reactivity with H atoms, in converting N-2 process gas into reactive NHx (x = 0-3) radical species.