When emptying isothermally a supercritical gas container, as already well known, the cumulative production varies linearly with the pressure to compressibility factor ratio. However, the instantaneous production (that is to say, the instantaneous standard volume of the released gas) varies in a totally different manner when looked as a function of pressure; in particular, at any temperature above the critical, this production goes through a maximum which is more and more pronounced when approaching the critical point. The locus of these maxima defines in pressure-temperature coordinates a curve which starts at the critical point, goes through a maximum and ends at the Boyle temperature. This new characteristic curve is analyzed and some rather surprising features are presented in this paper. For pure component systems, this curve coincides with the locus of maximum density fluctuations. (C) 2009 Elsevier B.V. All rights reserved.