The Rietveld refinement approach has been applied to semi-crystalline materials in the case of bulk metallic glasses. Specifically, the crystallisation behaviour of a Fe48Cr15Mo14Y2C15B6 bulk metallic glass has been investigated by in situ X-ray diffraction obtained using a high intensity monochromatic synchrotron beam at the ID11 line of ESRF in Grenoble, France From the Rietveld analysis of diffraction patterns, adapted to include a paracrystal model for describing the amorphous metal component, volume fraction of evolving phases. structure and microstructure parameters of nucleated phases have been followed as a function of run number, which turns out to be related to the applied temperature Capabilities and limitations of the approach are discussed in terms of structural information.