This study describes the effect of multiwall carbon nanotubes MWCNT as an additional C-source on the structural and morphological features of silicon oxycarbide ceramics obtained by different heat treatment times of a hybrid polymeric precursor at 1500 degrees C under an Ar atmosphere The precursor was prepared by a hydrosilylation reaction from poly(methylsiloxane) and divinylbenzene The presence of MWCNT promoted a more effective crystallization of the SIC and c-SiO2 phases in the resulting ceramics in comparison with those without MWCNT The heat treatment time at 1500 degrees C also affects the crystallization of the ceramic product In addition an Increase of the defect density in the Csp(2) sheets of the C-free dispersed phase together with higher disordering of this phase were observed in the ceramics obtained from MWCNT-containing precursor The presence of MWCNT resulted in a ceramic matrix with a different morphology in relation to that observed in the absence of MWCNT with lower resistance to oxidation Moreover the extra C-source present in the ceramic matrix introduced as MWCNT reduced the solubility of SiO2 in this matrix promoting the early segregation of SiO4-rich sites Consequently a more effective crystallization of SiO2 with relatively large c-SiO2 crystals and of beta-SiC occurred in the material with the increase of the heat treatment time (C) 2010 Elsevier B V All rights reserved