the vapor deposition of butadiyne, c/sub 4/h/sub 2/, has been investigated as a novel approach to the application of surface treatments and coatings on carbon fibers to promote adhesion to matrix resins. a thin adherent polymer coating is formed on carbon fibers when mildly heated (25 to 100 degrees c) in an atmosphere of butadiyne monomer. this coating is characterized by an elemental composition similar to that of the monomer, a high density of pendant terminal acetylenic groups and a noncrystalline featureless film morphology. to investigate this coating as an adhesion promoter, single fiber/resin tensile tests were conducted using hercules as-4 and hms-4 carbon fibers encapsulated in a bisphenol a epoxy/m-phenylene diamine resin. for the as-4 fiber with 3 levels of polybutadiyne deposition (0.2, 1.1 and 9.4 percent by weight), significantly shorter critical fiber lengths (0.30, 0.29 and 0.26 mm respectively) were obtained in comparison with the uncoated control test (0.40 mm). it is unclear whether this result is caused by an increased interfacial adhesion or a decreased fiber tensile strength. the polybutadiyne deposition onto the hms-4 fiber did not promote an enhanced adhesion in comparison with the control experiment