Several interlaboratory tests according to ISO 5725 (1994) on polymeric materials using thermal analysis are presented. The results, especially the repeatability - and reproducibility limit of the data will be discussed. Thermogravimetric analysis (TGA) was used for the determination of carbon black, ash and plasticizer content in polymeric compounds (thermoplastic and rubber materials). Additionally the determination of the content of vinyl acetate in ethylene/vinyl acetate copolymers was performed with TGA. Differential scanning calorimetry was used for the determination of the cristallinity of thermoplastic materials, the curing of epoxy resins and the oxidation induction time of polyolefins. Some surprising results were obtained.