We have discovered two new miscible polymer blend systems, namely, Torlon (R) 4000T with Matrimid (R) 5218 and Torlon 4000T with polybenzimidazole (PBI). Both Matrimid (R) 5218 and PBI are miscible at a molecular level with Torlon 4000T over the whole composition range as confirmed by microscopy, DSC, FTIR and DMA. DSC and DMA studies show the existence of a single glass transition in each blend. The T-g-composition curve of Torlon/Matrimid blend system forms a sigmoid curve as a function of composition, while the T-g-composition curve of the Torlon/PBI blend system is double parabola-like. FTIR spectra show the existence of hydrogen-bonding interactions in these two polymer blend systems. (C) 2007 Elsevier Ltd. All rights reserved.