The glass-transition temperatures (Tg) of some polymers reported in the literature have always been a source of great uncertainty. The values reported for poly(methacrylonitrile) (PMAN, 100 and 120 degreesC) are well above the value determined in this study (67 degreesC). It is clearly shown by FTIR and DSC work that formation of cyclic structures during the drying of PMAN, even at low temperatures, is the main reason for the high T-g values observed. The contributions of naphthydrine type cyclic structures and intermolecular crosslinks in the increase of the T-g are determined over an aging temperature interval of 90-300 degreesC. The combined effects of intra- and intermolecular linking cause an increase in the T-g from 67 to 116 degreesC. The hardness measurements also confirm the value determined by DSC.