This document describes the results of an investigation on the variation of the so-called fundamental derivative of gas dynamics, Gamma, in the vapor-liquid critical region of well-measured substances, namely methane, carbon dioxide and water, for which accurate, scaled fundamental equations are available. The results demonstrate that for a pure fluid in the single-phase thermodynamic regime, Gamma diverges to +infinity independent of the direction of approach of the vapor-liquid critical point. Furthermore, in the two-phase thermodynamic regime, Gamma diverges to -infinity independent of the direction of approach of the vapor-liquid critical point. These two qualitative results, as well as the value of the exponent giving the power-law dependence of Gamma along the critical isochore as a function of vertical bar T - T-c vertical bar/T (T is the temperature and "C" indicates its critical point value), namely approximate to-0.89, are similar for all pure, non-ionized fluids belonging to the class of 3-dimensional Ising-like systems, i.e., systems governed by short-range forces. (C) 2012 Elsevier B.V. All rights reserved.