The solid-state structure and transition properties of three poly(N-methylcyclodisilazanes) were studied using differential scanning calorimetry (DSC), infrared (IR) and nuclear magnetic resonance (NMR) spectroscopies, dielectric relaxation, and X-ray diffraction techniques. Two endothermic transitions were observed in each of the DSC curves. It was found that the first transition, at lower temperatures, involves a change from a three-dimensional ordered phase to a two-dimensional ordered phase. This was shown by the disappearance or broadening of the intramolecular X-ray reflections, but also by the IR and NMR results, which show that the local mobility of the chain backbone and of several substituents increases significantly above the first transition temperature. This behavior can be achieved when going from a 3-D order where the chains are fixed in space one relative to the others to a 2-D order where the chains can move easily along the c axis. The second transition, at higher temperatures, is a melting transition according to X-ray and polarized microscopy results. The transition properties of the alternating copolymer are similar to those of the corresponding asymmetric homopolymer but its X-ray spectrum is close to that of the symmetric motif.