In the thermal imidization of an alternating inorganic organic hybrid network, there is an inverse relationship between the length and flexibility of the organic bridges and the extent of the layer shrinkage. The hybrid Material studied here consists of,polyhedral oligomeric silsesquioxanes that are covalently bridged by amic acid groups. During heat treatment, shrinkage of the materials occurs due to the removal of physically bound water, imidization of the amic, acid groups, and silanol condensation: For five different bridging groups with different lengths and flexibilities, comparable mass reductions are observed. For the shorter bridging groups, the dimensional changes are hindered by the limited network mobility. Longer, more flexible bridging groups allow for much greater shrinkage. The imidization step can be,described by a decelerating reaction mechanisin with an onset at 150 degrees C and shows a higher activation energy than in the case of entirely organic polyimides. The differences in the imidization kinetics between hybrid and purely organic materials demonstrates the need for close study of the thermal processing of hybrid, hyper-cross-linked materials.