A carbon ribbon has been produced from a polyimide film whose microstructure can be varied by the degree of restraint or stretch during pyrolysis. The microstructure was varied from a coarse lamellar structure to a welldefined lamellar structure typical of a highly ordered graphite. Increasing the degree of stretch to high values transforms the lamellar to a non-lamellar structure. Subsequent to graphitization, the lamellar structure tensile strength approaches 300,000 psi while its modulus can be as high as 130 x lo6 psi. Transformation to the nonlamellar structure causes a decrease in modulus to approximately 40 X lo6 psi. *This work was supported by the United States Atomic Energy Commission. 128. Development and properties of PAN-based carbon felt-pyrolytic carbon composites* H. 0. Pierson, D. A. Northrop and J. F. Smatana (San&a Laboratories, Albuquerque, New Mexico). The properties of carbon felt-pyrolytic carbon matrix composites based on polyacrylonitrile (PAN) and viscose rayon felt precursors are compared. The PAN-based composite has higher strength, modulus and thermal conductivity, and a significantly reduced thermal expansion. A thermal stress resistance parameter for the PAN-based composite is approximately three times greater.