The amorphous-to-crystalline transformation behavior of Fe48Cr15Mo14Y2C15B6 bulk metallic glasses was first investigated by high-temperature differential scanning calorimetry. Three events were detected with onset temperatures at 922, 975, and 1036 K, respectively. In situ synchrotron radiation x-ray diffraction patterns were collected during continuous heating and analyzed with the Rietveld approach. To describe simultaneously the amorphous fraction and crystallization products as a function of temperature, a paracrystalline structure-factor model was developed. It was included for quantitative evaluation of the amorphous phase, together with the structure factor of Cr23C6- and Fe3Mo3C-type phases observed during crystallization. Volume fractions of phases as well as lattice parameters, average lattice disorder, and crystallite size of the crystallized phases have been followed as a function of temperature. In this way, the structure evolution and thermal events have been closely inspected and accounted for by a crystallization mechanism.