This Communication describes a new thermal desorption/pyrolysis vacuum-assisted plasma ionization (pyro-VaPI) ion source coupled to ion mobility-mass spectrometry (IM-MS) for insoluble polymer analysis. Pyro-VaPI combines a pyrolysis device, soft ambient plasma ionization, IM, and MS into a single platform for polymer analysis with minimal sample preparation. Nylons, a widely used and well-studied thermoplastic, were chosen to evaluate the pyro-VaPI performance. Six different nylon polymers were studied and characterized. With the application of IM-MS, two different isobars for the protonated cyclic dimers of 6-6, 6-9, 6-10, and 6-12 nylon and two isobars for the cyclic tetramer of nylon-6 were detected at 200 °C. These isobars were observed at different heating times, with the species drifting faster in the IM cell appearing several minutes after the slower drifting species. To the best of our knowledge, these isobaric dimers and tetramers have not been previously reported, indicating that pyro-VaPI IM-MS is a useful tool for the structural characterization of heated or pyrolyzed polymers.