EuB6-xCx carbide borides with a CaB6-type structure were prepared in the range 0 ≤ x ≤ 0.20. Magnetic and transport measurements performed on single crystals show that pure EuB6 is ferromagnetic below Tc = 13.7 K. It is semiconducting in the paramagnetic region with an intrinsic small band gap estimated to be 0.05-0.1 eV; an electronic transition to a semimetal occurs below Tc. The carbon-doped EuB6 samples are degenerate semiconductors in which both the carrier concentration and the antiferromagnetic interactions increase with carbon content. These carbide borides were studied using electron spin resonance in the X band at temperatures above the magnetic ordering temperature. A comparison at 300 K of the peak-to-peak linewidth ΔHp-p (the first derivative of the Eu2+ resonance line) in both the Eu2+B6-xCx and the Eu2+ 1-xGd3+ xB6 systems shows that only x 2 electrons are delocalized in the conduction band for x carbon atoms present in the unit cell. This observation is well confirmed by Hall effect measurements. Furthermore a weak and extremely narrow line which appears (superimposed on the Eu2+ resonance line) only when x ≠ 0 involves an electronic localization for the remaining x 2 electrons introduced. They are likely to form pairs. © 1981.