This manuscript presents the results of a research project dealing with the processing of microcellular foams from molecular composites. The main goal of this work is the preparation, characterization, and evaluation of polymer-based foams. A variety of thermoplastic polymers have been evaluated with regard to the extent to which they can be formed into monolithic foams, and methods have been developed for toughening these foams. The primary mechanism for improving toughness and other mechanical properties is inclusion of rigid-rod polymers as dispersed phases. Both the thermoplastic host material and the molecular composites have been processed into the desired foams through extensions of the proprietary methods developed by Wright Materials Research Company. Part I of this series of papers presents the results of polybenzobisthiazole(PBZT)/ polyetherketoneketone(PEKK) microcellular foams. The results show that the incorporation of the oriented rigid-rod polymers into a host thermoplastic matrix has given large improvements in mechanical and thermal properties of the foam. The properties of primary interest for optimization are of a mechanical nature, and for the anticipated structural applications include modulus, ultimate strength, maximum compressibility, impact resistance, and toughness. The microstructural details of the foams, which were monitored via standard microscopic techniques, were correlated with some of their properties. (C) 2003 Kluwer Academic Publishers.