Composite sheets and nanotubes of different morphologies containing carbon, boron, and nitrogen were grown in the electric are discharge between graphite cathodes and amorphous boron-filled graphite anodes in a nitrogen atmosphere. Concentration profiles derived from electron energy-loss line spectra show that boron and nitrogen are correlated in a one-to-one ratio; core energy-loss fine structures reveal small differences compared to pure hexagonal boron nitride. Boron and carbon are anticorrelated, suggesting the substitution of boron and nitrogen into the carbon network. Results indicate that single-phase CyBxNx as well as separated domains (nanosize) of boron nitride in carbon networks may exist.