1H-15N-13C correlation NMR spectroscopy techniques developed to identify and characterize co-polymer fragments in melamine-urea-formaldehyde (MUF) and phenol-urea-formaldehyde (PUF) model systems have been applied to industrially prepared MUF, PUF, and phenol-melamine-formaldehyde (PMF) resins. The NMR data confirm that co-polymers form in a commercially prepared MUF resin manufactured by Momentive Specialty Chemicals Pty. Ltd. Spectra from PUF model reactions were compared with those from a PUF resin and it was determined that PUF co-polymers did not form in the resin prepared using typical temperature and pH. Finally, NMR spectroscopy was used to identify and characterize PMF co-polymer bonds in a phenol-melamine-urea-formaldehyde (PMUF) resin prepared using a procedure from Momentive Specialty Chemicals Pty. Ltd. With these NMR techniques in hand, it is now possible to relate co-polymer structures to properties of commercial thermosets. (c) 2012 Wiley Periodicals, Inc. J. Appl. Polym. Sci., 2013