The thermal behaviour near the glass transition temperature T-g was studied for poly(styrene-co-maleic anhydride) or (SMA) copolymers with a broad range of molecular weight (M-w = 5500-180,000 g/mol) and amount of maleic anhydride (22-50 mol%). The influences of molecular structure for low- and high-molecular weight copolymers were detailed by using thermogravimetric analysis (TGA), differential scanning calorimetry (DSC) and temperature modulated differential scanning calorimetry (TMDSC). Data from TGA provide evidence for variations in thermal stability, mainly over the first degradation step up to 350 degrees C. Most interesting data results from TMDSC for high-molecular weight SMA, where the transition in reversible heat flow and Delta c(p) remains almost constant, while the transition in non-reversible heat flow and Delta c(p) gradually increases with amount MA. Comparing low- and high-molecular weight SMA, a linear model for T-g cannot be applied for the global amount of maleic anhydride, while it can be successfully implemented if a more detailed molecular structure with two constituents is considered. As such, further evidence is provided for the intrinsic heterogeneous molecular structure of high-molecular weight SMA with "styrene-rich" and "maleic-anhydride-rich" polymer segments. (C) 2014 Elsevier B.V. All rights reserved.