In this paper we describe the experimental techniques, the method of data analysis and the results of our recent structural studies for expanded fluid Hg. The energy-dispersive x-ray diffraction measurements were carried out for the first time in the density range from 13. 55 to 6. 8 gcm^-3 including the metal-nonmetal transition region. For the measurements we have developed a single-crystalline sapphire cell and a high-pressure vessel of our own design and improved the method of the data analysis. It was found that the volume expansion of liquid Hg in the metallic region, to the density of about 10gcm^-3, is not accompanied by a uniform increase of interatomic distance r₁ but mainly caused by a decrease of the coordination number N_1- When the transition region is approached, the decrease of N₁ becomes rather small and r₁ starts to elongate.