Ex-rayon carbon yarns have been prepared according to an original route characterized by a fast pyrolysis step in the presence of an organosilicon compound. The evolution of the mechanical and physicochemical properties of the fibres throughout the transformation of cellulose into carbon showed that the organosilicon additive was necessary to obtain carbon yarns exhibiting satisfactory mechanical behaviour and to improve significantly the fracture properties of the ex-rayon carbon fibres. (c) 2006 Springer Science + Business Media, Inc.