Amorphous boron-carbon-nitrogen (B-C-N) films with low density are potentially interesting as alternative low-dielectric-constant (low-κ) materials for future electronic devices. Such applications require deposition at temperatures below 300 °C, making plasma chemical vapor deposition (plasma CVD) a preferred deposition method. Plasma CVD of B-C-N films is today typically done with separate precursors for B, C and N or with precursors containing B–N bonds and an additional carbon precursor. We present an approach to plasma CVD of B-C-N films based on triethylboron (B(C2H5)3) a precursor with B-C bonds in an argon-nitrogen plasma. From quantitative analysis with Time-of-Flight Elastic Recoil Detection Analysis (ToF-ERDA), we find that the deposition process can afford B-C-N films with a B/N ratio between 0.98 and 1.3 and B/C ratios between 3.4 and 8.6 and where the films contain between 3.6 and 7.8 at. % H and 6.6 and 20 at. % of O. The films have low density, from 0.32 to 1.6 g/cm3 as determined from cross-section scanning electron micrographs and ToF-ERDA with morphologies ranging from smooth films to separated nanowalls. Scaning transmission electron microscopy shows that C and BN does not phase seperarte in the film. The static dielectric constant κ, measured by capacitance–voltage measurements, varies with the Ar concentration in the range from 3.3 to 35 for low and high Ar concentrations, respectively. We suggest that this dependence is caused by the energetic bombardment of plasma species during film deposition.