The double retrograde vaporization in binary fluid mixtures under high pressure is studied by means of the Gibbs-Konowalow equations. It is shown that the phenomenon is related to the behavior of two functions, the slope of tic lines in a volume-mole fraction diagram and the composition derivative of the vapor volume, which in turn depends on the partial molar volume of the solute. Due to mathematical constraints these curves can intersect more than once, thus creating double retrograde behavior. (C) 2003 Elsevier Science Ltd. All rights reserved.