Low-dielectric-constant SiOC(-H) films were deposited on p-type Si (100) substrates by using plasma-enhanced chemical-vapor deposition (PECVD) from triethoxysilane (TES; C6H16O3Si) and oxygen gas as precursors. Factors affecting the carbon and hydrogen incorporation into the SiO2 films during plasma-enhanced chemical vapor deposition were studied for different experimental parameters by using an in-situ residual gas analyzer (RGA), optical emission spectroscopy (OES) and Langmuir probe measurements. Detailed information of the relative radical densities, the electron density (Ne) and the electron temperature (Te) of the bulk plasma was obtained in order to better understand the deposition process of the low-k SiOC(-H) thin films. The silane-to-oxygen (precursor flow rate) ratio in the process gas mixture and the radio-frequency (rf) power determine the chemical nature of the substrate surface and the incorporation rates of Si-O-C and Si-O-Si into the film. We interpret in detail the evolutions of plasma discharge parameters in terms of the variations in the composition of the plasma.