Polyimides (PIs) are a group of widely used synthetic materials that service a variety of different purposes including microelectronics, insulating films and aerospace applications. Depending on the requirements (defined by the particular final product), the actual composition of PIs may show substantial chemical variation. To study this variation in chemical structure, CE-MS can be employed for the determination of PI composition following chemical degradation of the polymer sample. PI is chemically decomposed to corresponding aromatic diamine and carboxylic acid components using an alkali fusion reaction. Solid polymer samples are fused in a potassium hydroxide melt yielding reaction products that are diluted in acid and can be immediately analysed by CE coupled to a Q/TOF-MS with quantification performed using conventional UV detection. This approach involves a simple and rapid sample preparation yielding both qualitative and quantitative information regarding the chemical composition of the polymer. Application of the CE-MS approach is shown for a range of commercially available PI and poly(amideimide) materials and the results are used to infer the respective chemical compositions.