The properties of fiber-reinfored cement-based composite materiais are dependent on the charactertstics of the fiber, the motrix, and the fiber-matrix interface. In general, the nature and behavior of the fiber and matrix are reasonably well understood, but those of the interface are known in considerably less detad. Results of an experimental program demonstrate that the addition of an acrylie polymer to fiber-reinforaed mortar and cement strengthens the matrix material and improves the structure of interface, thus increasing the interface bond strength.Tensile tests, fiber pullout tests. microhardness studies, and electron microssopy studies were conducted on mortar matrix materials. with and without the addition of the acrylic polymer. The tensile strength of the matrix material, the interfacial bond strength between the matrix and steel fibers, and the energy required for fiber debonding and pullout were incensed by a factor of almost four with the addition of 15 potent acrylic polymer by weight of cement. TO explain these incenses, the microhardness and microstructure of an annular region of the composite surrounding a fiber (interface transition ring) were investigated. Addition of the aeryhc pollymer to a cement matrix resulted in increases in micrahardness of the cement matrix material of the same order of magnitude as increases obtained in tensile strength, bond strength, and energy required for fiber debonding and pullout. Observation with a scanning electron micro-scope indicated that cracking along the fiber-matrix interface (before loading) is substantially reduced by the addition of acrylic polymer. Possible explanations for this are a reduction in the film of water that surrounds the fiber and the filling of small cracks with the acrylic polymer material itself.