This investigation has involved the use of '3C/'2C isotope ratio mass spectrometry to study the oxidative gasification of carbon fibre reinforced composites manufactured from coal tar pitch mesophase fibres and petroleum pitch matrix, and carbon electrodes manufactured from petroleum coke and coal tar pitch binder. Comparisons between the composites and their components are shown to be valid only when the components have been prepared under the same carbonization conditions. In the case of partial gasification studies, the '3C/'2C isotope ratios for the composites and the components vary with the gasification temperature, and therefore for valid comparisons to be made, gasification studies of the components and composites must be carried out under the same conditions. Comparison of the carbon isotope ratio studies with conventional thermogravimetric studies on the composites and their components shows that the reactivity of the binder phase, in the temperature range where the reaction is under chemical control, is enhanced in the composite compared with the components but the apparent activation energy is similar. Furthermore the use ofthermogravimetric reactivity data for the composite and its components may be misleading because interfacial phenomena occurring during the carbonization process, modify the structure. and reactivity of the binder phase. It is apparent that the carbon isotope ratio technique can be used to study the reactivity of carbon-carbon composites. However, considerable care is required to ensure that the comparisons between the composites and the components are valid by ensuring as far as possible identical carbonization and gasification conditions.