BCN nanotube brushes have been obtained by the high temperature reaction of amorphous carbon nanotube (a-CNT) brushes with a mixture of boric acid and urea. The a-CNT brushes themselves were obtained by the pyrolysis of glucose in a polycarbonate membrane. The BCN nanotubes have been characterized by EELS, XPS, electron microscopy, Raman spectroscopy and other techniques. The composition of these nanotubes is found to be BC4N. The nanotubes, which are stable up to 900 °C, are insulating and nonmagnetic. They exhibit a selective uptake of CO2 up to 23.5 wt%. In order to understand the structure and properties, we have carried out first-principles density functional theory based calculations on (6,0), (6,6) and (8,0) nanotubes with the composition BC4N. While (8,0) BC4N nanotubes exhibit a semiconducting gap, the (6,0) BC4N nanotube remains metallic if ordered BN bonds are present in all the six-membered rings. The (6,6) BC 4N nanotubes, however, exhibit a small semiconducting gap unlike the carbon nanotubes. The most stable structure is predicted to be the one where BN3 and NB3 units connected by a B-N bond are present in the graphite matrix, the structure with ordered B-N bonds in the six-membered rings of graphite being less stable. In the former structure, (6,0) nanotubes also exhibit a gap. The calculations predict BC4N nanotubes to be overall nonmagnetic, as is indeed observed. © The Royal Society of Chemistry.