Glass-ceramic composites like low-temperature cofired ceramics are densified at temperatures below 900 degrees C via liquid-phase sintering. For high-frequency applications, materials with low dielectric losses are needed. At these frequencies glass exhibits higher dielectric losses than a crystalline phase; therefore, the glass amount is reduced after densification by crystallization. To perform a quantitative X-ray analysis of such glass-ceramic composites, a method based on Rietveld refinement was developed. For optimum results, the X-ray diffraction patterns were fitted using the software TOPAS 2.1 from Bruker AXS. With this method, it is possible to determine quantitatively the amount of crystalline phases and also the amount of the remaining amorphous phase.