The chemical substitution of amine nitrogens in cured, N-15-labeled epoxy resins has been determined by a combination of rotational-echo double-resonance C-13 NMR and dipolar rotational spin-echo N-15 NMR. Amine-nitrogen substitution is at least 90% (that is, no more than 10% of all nitrogens have a directly bonded hydrogen) for resins formed from stoichiometric amounts of epoxide and either hexamethylenediamine or a 1:3 molar mixture of hexamethylenediamine and hexylamine. This direct measure of curing and cross-linking is consistent with indirect Fourier-transform infrared estimates of curing based on the disappearance of the deformation band of the epoxide ring.