Infrared tunable diode laser absorption spectroscopy (IR TDLAS) technique [1,2] has been used in a high pressure microwave (2.45 GHz) stainless steel reactor dedicated to diamond films deposition for electronic applications. Line integrated absorption measurements have been performed in H2/CH4 and H2/CH4/B2H6 plasmas in order to monitor hydrocarbon species (CH3, CH4, C2H2, C2H4 and C2H6) and diborane (B2H6). The densities of the detected species were in the range of 1012 - 1017cm-3. Parametric studies as a function of pressure-microwave power couple and gas composition (admixtures of methane and diborane) have been carried out on wide ranges of experimental conditions, especially for high pressure (³ 200 mbar) and high power (³ 3 kW). The results show an increase of the degree of dissociation of CH4 and B2H6 with pressure and power. C2H2 is the most abundant reaction product. The concentration of CH3 within the plasma bulk was detected to about 1013 cm‑3 and only for the lowest power density conditions. In the presence of methane the concentration of diborane is below the limit of detection of about 5*1011cm‑3.