We review recent work on the acoustic characteristics of mercury in molybdenum and niobium cells at high temperatures (to 1900 K) and high pressures (to 180 MPa) and report some further experimental progress. The sound velocity, and the transmitted and reflected signals have been observed simultaneously. Apparent first-order phase transitions appear in the vapor region of the mercury phase diagram. The corresponding transition curve (for Mo) meets the bulk liquid-gas coexistence curve tangentially at (T-0, P-0)congruent to(1250 K, 30 MPa) and terminates above 1860 K and 183 MPa. The data provide strong evidence that the observations represent prewetting transitions, bounded below by a wetting transition at(T-W, P-W)drop(T-0, P-0) and above by prewetting critical points which lie well above the bulk critical point (T-c congruent to 1764 K, P-c congruent to 167 MPa). (C) 1998 Elsevier Science B.V. All rights reserved.