In situ vitrification in a synchrotron beam during cooling of a Pd(42.5)Cu(30)Ni(7.5)P(20) melt allowed continuous acquisition of X-ray diffraction spectra in the supercooled liquid region between the liquidus temperature T(I) and the glass transition temperature T(g) for the first time in a metallic liquid alloy. These and subsequent acquisitions on reheating allowed determination of the structural changes in the supercooled and glassy states using the real space atomic pair distribution function PDF(R) and radial distribution function RDF(R). The nearest neighbor (NN) distance in the first coordination shell has a low R peak (pre-peak) that appears and increases on cooling from the liquidus temperature to the glass transition temperature in the supercooled liquid region. In this temperature range reversible expansion of the NN distances in the first coordination shell is observed. The slope of the expansion changes discontinuously at the glass transition. This behavior is explained by the theory of local topological fluctuations. (C) 2010 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.