Ultrathin B–C–N layers grown on Ti substrates are investigated as efficient anodes for electrochemical water splitting. A fast and direct synthetic route has been used based on plasma-enhanced chemical vapor deposition with methylamine borane as a single-source molecular precursor. The effect of growth time on the morphological and structural properties and on the chemical composition of the layers has been investigated by scanning electron microscopy, Raman spectroscopy, X-ray photoelectron spectroscopy, and transmission electron microscopy coupled with electron energy loss spectroscopy. Flat B–C–N layers on top of an amorphous titanium oxide layer present at the Ti surface have been obtained by using short growth times, while longer growth times give rise to core/shell structures formed by vertical wall B–C–N layers and titanium carbonitride phases. The obtained layers present enhanced electrocatalytic activity for the oxygen evolution reaction in alkaline aqueous solutions. Moreover, because of their ultrathin nature, the B–C–N layers preserve the photocurrents of the underlying titanium oxide layer, acting as transparent electrodes with high conductivity for the photogenerated charge carriers and improved electrocatalytic activity for the oxidation of water to oxygen gas.