Single fiber model composites of polyvinyl alcohol (PVA) fibers as well as alkali resistant (AR-) glass fibers and a cementitious matrix were used to investigate the adhesion strength under quasi-static and high-rate loading in a pull-out test. Differently sized AR-glass fibers were spun for fundamental understanding the effect of the fiber/matrix interphase on the strain hardening. As a first approach, the fiber surface was modified in oppositional ways using the following sizings: one based on a polypropylene (PP, weak) film former and another one based on a styrene-butadiene (SB, strong) film former. This unique set of data allows the comparison of the force-displacement curves during pull-out depending on the applied sizing, the embedded length and the loading rate. A significant influence of the sizing on the frictional behavior after debonding was found. In the case of PVA fibers, the interfacial behavior of as-received (finished) fibers and fibers from which the finish was removed by extraction in solvents (ethanol and n-hexane) was compared. It was shown that PVA fibers in concrete are highly strain rate dependent, whereby the fiber surface treatment controls the ratio between completely pulled out fibers and sheared-off fibers.