three distinct families of composites were studied in order to structurally characterize their fiber-matrix interface and interphase. the first group consisted of xau/5245 (untreated fibers), xas/5245 (oxidized fibers), xau/eda/5245 and xau/teta/5245 (amine-treated fibers). the second group included im7/c59 and im7/isoh/c59 (oiled fibers). the third group consisted of t300/5/da508 (epoxy-oiled fibers) and t300/elastomer/da508 (elastomer-oiled fibers). the first group illustrated the effect of surface treatments on the interface's structure and thus on the composite's mechanical properties. the second group showed the usefulness of oiling in promoting fiber/matrix cohesion. two types of oiling were compared in the third group, making it possible to understand the effect of fabrication conditions on the quality of oiling. all the observations clearly showed the importance for the fibers' mechanical qualities of good load transfer from the matrix to the fiber and vice-versa. the quality of the transfer depends on the interface's resistance to shear, which is a direct function of fiber-matrix adhesion. optimizing, though not necessarily strengthening, fiber-matrix adhesion could produce improvements