In this article an expert system called PHYP (PHYsical Properties) is described which aids engineers in the selection of an appropriate vapour-liquid equilibrium (VLE) method when performing various process calculations. Test examples demonstrate the ability of the expert system to solve problems from the field of phase equilibria in the shortest way without redundant questions to the user giving the results expected. PHYP was compared with the same purpose expert system CONPHYDE when solving, the same classification problems. For final solution, PHYP needs less user＇s assertions because of its smaller and more efficient decision tree. The expert system PHYP was generated by an automatic expert system shell, Assistant Professional. This way of building expert systems is much more efficient than development of a classic expert system like CONPHYDE which needs decision rules formulated by hand to be built into its inference network.