Methane and CO2 are the main components of biogas; therefore its direct conversion into a higher added value gas as syn-gas (mixture of CO and H-2) is a very interesting alternative for the valorisation of such renewable resource. In this work, firstly a thermodynamic analysis of the decomposition of CH4:CO2 mixtures at different temperatures and CH4:CO2 ratios simulating the biogas composition, has been carried out. Secondly, the decomposition of a mixture with a molar ratio of 1: 1 has been studied in a fixed-bed reactor by using a Ni/Al2O3 based catalyst, at the temperature range in which according to the thermodynamic study, carbon formation is favoured. Results obtained have been compared to those of methane decomposition carried out under the same experimental conditions. Co-feeding of CO2 and CH4 avoids catalyst deactivation substantially, allowing to obtain a syn-gas with H-2:CO ratio close to 1. Moreover, the carbon obtained from mixtures of CH4 and CO2 is deposited as fishbone carbon nanofibres at 600 degrees C and ribbon carbon nanofibers at 700 degrees C, both being materials with high added value which can be used in multiple applications. (C) 2011 Elsevier Ltd. All rights reserved.