Tunable infrared diode laser absorption spectroscopy has been used to detect the methyl radical and three stable molecules, CH(4), C(2)H(2) and C(2)H(6), in radio frequency plasmas (f = 13.56 MHz) containing hexamethyldisiloxane (HMDSO). The methyl radical concentration and the concentration of the stable hydrocarbons, produced in the plasma, have been measured in pure HMDSO discharges and with admixtures of Ar, while discharge power (P = 20-200 W), total gas pressure (p = 0.08-0.6 mbar), gas mixture and total gas flow rate (Phi = 1-10.9 sccm) were varied. The methyl radical concentration was found to be in the range of 10 13 molecules cm(-3), while methane and ethane are the dominant hydrocarbons, with concentrations of 1(14)-10(15) mol cm(-3). Conversion rates to the measured stable hydrocarbons (R(C) (C(x)H(y)): 2 x 10(12)-2 x 10(16) molecules J(-1) s(-1)) could be estimated in dependence on power, flow, mixture and pressure. Under the used experimental conditions a maximum deposition rate of polymer layers of about 400 nm min(-1) has been found.