Gas phase thermodynamic equilibrium calculations involving N2/B2H6/CH4/H2 mixtures were performed to investigate the chemical interactions leading to boron incorporation and microstructure variations in microwave plasma assisted chemical vapor deposition of diamond films. Molecular fractions of several BHx (x = 1-3) species were calculated to study the incorporation mechanism of boron atom into diamond structure. A strong influence of the BH in causing the boron incorporation level in diamond lattice is confirmed by the correlation of its modeled equilibrium composition in the gas phase with boron content as determined experimentally. Nitrogen addition leads to nanocrystallinity and a reduction in boron incorporation due to a decrease in BH as additional B/N/H radicals are formed in the gas phase. We also obtained a good degree of agreement between the theoretically predicted CH3/CN gas phase ratio and the experimental surface roughness trends as measured for all samples. © 2008 Elsevier B.V. All rights reserved.