The chemical structure of amino adhesives produced by the strongly acid process was investigated by C-13 Nuclear Magnetic Resonance spectroscopy. This technique allowed the identification of functional groups and its quantitative determination. Concentration of cyclic polymeric structures (urons) was shown to be related with adhesive performance and with particleboard physico-mechanical properties and formaldehyde content. Higher urons concentration presented lower viscosity and reactivity. Particleboards produced with resins with lower urons concentration presented lower formaldehyde content, but also lower internal bond strength. Wood-based panels produced fulfilled E1 class requirements for formaldehyde emissions, indicating that strongly acid process is an alternative to the conventional alkaline-acid process. (c) 2013 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 130: 4500-4507, 2013