The oxidation behaviour of partially carbonized polyacrylonitrile fibres was studied primarily by both dynamic and isothermal thermogravimetric analysis. These fibres, referred to as quasi-carbon fibres (QCFs), were obtained by pyrolysis of a polyacrylonitrile precursor at a heat-treatment temperature (HTT) ranging from 400-950 degrees C. Results indicated that QC fibres exhibited increased thermal stability with increasing HTT. The oxidation behaviour was strongly related to the microstructure of QC fibres. An empirical model was developed to simulate the real oxidation process. Two different oxidation mechanisms were operational in QC fibres; one with a more moderate oxidation rate and the other with an auto-acceleration effect. Only the QC fibres that were heat treated above 650 degrees C to develop an extended two-dimensional graphite-like structure, exhibited an auto-acceleration effect.