Novel poly(imide siloxane) copolymers were prepared and developed as high performance fiber coatings. These copolymers were synthesized and characterized extensively as a function of chemical composition. The polyimides were then utilized to coat prototype fibers which were subsequently evaluated regarding thermal stability, thermal behavior, and hydrophobicity. The polymer series included poly(imide siloxane)s which were prepared in two steps, the first involving the generation of soluble poly(amic acid} intermediates which were then cyclodehydrated by heating in a coamide solution, at temperatures ranging from 140 to 170°C, resulting in soluble polymer which exhibited excellent thermal and mechanical properties. Because of the different nature of the imide and siloxane, a two phase microstructure developed at relatively low block molecular weight. X-ray photoelectron spectroscopy (XPS) and contact angle measurements indicated that the surface of the copolymer films was dominated by siloxane. A series of coating solutions was prepared, controlling the solution concentration, solvent, and viscosity. Polybenzimidazole and Kevlar aramid multifilament yarns were immersion coated, dried, and evaluated. Due to the surface segregation of siloxane, these coated fibers advantageously displayed lower moisture sorption as measured in three environments of different humidities. Thus, the coating acted as a hydrophobic barrier. These coated fibers were also evaluated by thermogravimetric analysis (TGA) which displayed that the dynamic thermo-oxidative stability was improved. Finally, thermal expansion coefficients were measured in order to determine coating integrity or matrix/resin integrity under thermal stress. / M.S.