The evolved gas analysis-mass spectrometry (EGA-MS) method is described as a new approach for determining the degree of imidization (DOI) in polyimide (PI) films. Partially imidized PI films allowed water to release through the re-imidization process at a sufficiently high imidization temperature. Evolved water from the re-imidization process was quantitatively detected by EGA-MS. From the obtained water content, the number of moles of residual amic acid groups in repeating units of PI was found. Consequently, the DOI of the PI films could be found from the mole ratio of PI and the sum of the PI and the residual polyamic acid (PAA). A water content of 0.018% and a DOI of 99.85% can be measured from 40 mg of PI films using this method. In this study, it was found that rigid PIs showed fast imidization reactions at relatively lower temperatures, while flexible PIs were activated and showed fast imidization reactions at relatively higher temperatures. In addition, the end point of the imidization process in multi-layer PI films was determined by this method. (C) 2012 Elsevier B.V. All rights reserved.