In 1978 Jean Vidal outlined an equation-of-state formulation for vapor/liquid-equilibrium calculations based on direct fitting of the energy parameter. There soon followed a paper by Huron and Vidal devoted to mixing rules, which included a brief review of the Vidal formulation under the heading "polynomial mixing rule." Neither paper presented examples of Vidal's formulation in any detail. Subsequent work focusing on mixing-rule models - Huron/Vidal, its variations, and Wong/Sandler - are treated extensively in the 1998 monograph by Orbey and Sandier. However, no mention is made there of Vidal's formulation, and indeed it seems never to have been evaluated in any detail. The purpose of this paper is to revive and exploit the Vidal formulation for general application to vapor/liquid-equilibrium calculations. It is theoretically sound, and at least as convenient and accurate in application as the gamma/phi formulation. (c) 2007 Elsevier B.V. All rights reserved.