AbstractPyrolysis mass spectrometry (Py‐MS), time/temperature resolved Py‐MS (TTR‐Py‐MS), and pyrolysis tandem mass spectrometry (Py‐MS/MS) were used to determine the structure of an additive present in Biomer and Lycra Spandex. The additive was found to form a second phase in the polymer matrix which was insoulble inN,N′‐dimethylacetamide. This insoluble portion (7% by weight) was separated by centrifugation. The structure of this additive is proposed to be poly(2‐diisopropyl amino ethyl methacrylate)(DPA‐EMA). The additive is present as a stabilizer for UV radiation and chloride and hypochlorite ion attack. A chloride containing compound of similar structure (2‐diisopropyl amino ethyl chloride) (CAS No. 96‐79‐7) is also detected. This chloride‐containing compound may be formed by reaction of the polymeric additive with chloride ions du