The glass transition and the phase formation during crystallization of the Zr52Ti5Cu18Ni15Al10 bulk metallic glass were followed in situ by high-temperature X-ray diffraction using synchrotron radiation at the high energy beam-line BW5 ( λ = 0.01040 nm) at the storage ring DORIS (HASYLAB, Hamburg). The experimental set-up enables to record intensities in transmission up to scattering vectors q of 200 nm -1 with a measuring time of 20 s per diagram. The crystallization starts with the formation of an extremely fine nanostructure followed by the transformation into tetragonal NiZr2-type crystals plus an unknown phase. Both phases are metastable and transform at about 1123 K into the stable equilibrium phases. The temperature dependence of the structure factor S(q) of the glass can be well described within the framework of the Debye theory. At the glass transition the first derivative dS(q)/dT changes. A Debye temperature Θ = 412 K was estimated for the glassy, and Θ = 162 K for the liquid state of the Zr52Ti5Cu18Ni15Al10 alloy. The short-range order of the glass, of the supercooled liquid state, and of the equilibrium melt at T = 1193 K is found to be quite similar.