In the present work, we report on the role of the methanol medium and ball-milling time in the substitution of carbon in carbon nanotube CNT-MgB 2 superconductors. In our samples, we find that the CNTs are intact and well dispersed. However, the liquid medium (methanol) used for dispersion of the constituent materials is also acting as a source of C for substitution. However, the substitution of C from methanol is not direct; rather, B 2 O 3 , which had been considered as just an impurity, plays a positive role in binding the methanol molecules to the surface of B. The detailed mechanism of methanol absorption and the role of B 2 O 3 and ball-milling time are presented here. In addition, we present the J C (H, T) data, which show that approximately 3% of C substitution provides the best critical current density at 20 K.