The propagation of damage in continuous fiber polymer matrix composites depends strongly on the fiber / matrix interface (interphase). Therefore it is critical to measure interface behavior. The recent application of micro-Raman spectroscopy (MRS) in the field of composites micromechanics has created a new era of experimental micromechanics and shed light on a number of crucial points. MRS, with the ability to measure local stresses with a spatial resolution of 2 mu m, allows very close monitoring of the interfacial behavior in composites including interfacial crack initiation and propagation. This paper will demonstrate the application of MRS on monitoring and understanding the effect of an interphase on the composite interfacial micromechanics. Namely, the effect of fiber sizing on thecomposite micromechanics will be presented and compared to micromechanical models and to results obtained using other interfacial measurements techniques. The properties of the created interphase, due to fiber sizing, and its relationship to the fiber volume fraction of the composites are also discussed.