Glas fiber reinforced polymers are increasingly being used in all kinds of applications. The main reason for this is that one obtains excellent mechanical properties (e.g. strength) at an acceptable price. The optimal use is achieved only if a reliable characterization can be performed of the material and of the components made out of it. Therefore nondestructive testing methods are important. It has been shown previously that microwaves are suited for defect characterization [1–3] and for the determination both of fiber content and orientation [4,5]. A detector arrangement with 4 detectors located along a rectangular waveguide responds to the refractive index and to the attenuation of the material at microwave frequencies. Therefore it has been used for defect characterization [6,7], and it is also applicable to determine fiber content: Figure 1 shows how phase and amplitude derived from the signals of the 4 detectors depends on fiber content.