The structure and molecular dynamics of self-condensation products of three different melamine resins based on hexa(methoxy methyl)melamine are studied by C-13 NMR in the solid state. The application of direct (DP) and cross-polarization (CP) pulse sequences shows that uncured and cured melamine resins are motionally heterogeneous systems, with the mobile and rigid parts consisting of the same basic structural units. Viscous lightly cured samples based on commercial melamine-formaldehyde resins contain low-molecular weight species which act as plasticizers. In these resins, three motionally different methyl groups are observed. Their existence is confirmed either by spectral deconvolution or by fitting the experimental signal intensities to the cross-polarization dynamics with the assumption of two cross-polarization-transfer rates. Fast cross-polarizing rigid methyl groups are accompanied with the spinning sidebands. On the basis of DP and CP relaxation measurements, quantitative results for three major structural units are calculated. Problems concerning the N-14-C-13 quadrupolar interactions and quantitative analysis are discussed.