Abstract: Polyacrylonitrile (PAN) copolymers are important precursors for making high performance carbon fibers. Two kinds of carbon fiber (CF) precursors based on acrylonitrile and two commonly used comonomers (itaconic acid and methyl acrylate, IA and MA) were prepared in this work via dipolymerization and ternary polymerization, respectively. The stabilization processes of these two PAN precursors were studied with thermogravimetry (TG) analysis and pyrolysis synchrotron vacuum ultraviolet photoionization mass spectrometry (Py-SVUV-PIMS). Given the TG results acquired under nitrogen atmosphere, three thermal decomposition stages were identified for the copolymer of PAN/IA, while only two stages were observed for the terpolymer of PAN/IA/MA. Both the first and the second stages for PAN/IA can be attributed to the scissions of the linear molecular chains of PAN. But they were induced in two different ways. As many studies already reported, with the addition of IA, both of radical and ionic cyclization reactions were proceeding simultaneously during the thermal stabilization process of PAN/IA. So, the first decomposition stage of PAN/IA was probably caused by the liberated heat of the radical cyclization reactions, which showed up in advance of the normal decomposition stage. In contrast, the decomposition of PAN/IA/MA at the first stage was conducting with a mild rate since the radical cyclization process was greatly restrained by the comonomer of MA. The mass spectra of decomposition products of these two CF precursors at isothermal temperatures were measured using Py-SVUV-PIMS. Thanks to the tunability of the photon energy of SVUV-PIMS, almost all of the pyrolysis products can be identified. Major pyrolysis products were divided into three categories, which are composed of nitrogen-containing compounds, acrylonitrile monomers and oligomers, and cyclized compounds. Temperature-evolved profiles of selected species under nitrogen and air atmosphere were also obtained during the programmed heating processes. Based on the formation profiles and yields of specified products, it can be concluded that the ordered molecular chain of PAN was damaged by the addition of MA and was finally converted into several amorphous segments. Since cyclization reactions tend to be initiated in amorphous regions of PAN, the thermal stabilization process of PAN can be greatly improved with the help of MA. Meanwhile, a relatively higher amount of methacrylonitrile was produced during the programmed heating process of PAN/IA/MA under air atmosphere. According to this, a possible formation mechanism of methacrylonitrile was proposed, which suggests that MA did not participate in the cyclization of PAN. In another word, MA is proved to be a neutral PAN comonomer. Furthermore, by comparing the evolution profiles of HCN, H₂O and CO₂ during the heating processes of these two CF precursors, we can conclude that the PAN fiber has a better carbon storage capability with the addition of both IA and MA.