A numerical model was used to examine experimental data in which a gas chromatograph measured the composition of gas sampled through a 70-μm orifice in the growth substrate during atmospheric-pressure RF plasma diamond CVD. Substantial discrepancies were found between the measurements and the predicted species mole fractions at the surface, but the data were in much better accord with predicted mole fractions ∼0.2 mm above the surface. This may possibly be due to the perturbation caused by the orifice. The model was also used to examine the predicted effects of freestream and substrate temperatures on the surface concentrations ratios of H, CH3 and C2H2. Different trends versus substrate temperature were found according to whether the freestream temperature was 3000 K or 4000 K.