I. INTRODUCTION Granular matter in general [1, 2], and granular gases in particular [3–5], have been recently the object of intensive theoretical and experimental research. Not in the least this is be- cause of the opportunity they offer to investigate fundamental questions of non-equilibrium physics. One of these issues is the nature and properties of non-equilibrium fluctuations and their relevance for the description of the macroscopic behavior of the system. The simplest fluctuations that can be considered are those of global properties of a system. The total energy fluctuations of an isolated granular gas, modeled as an ensemble of smooth inelastic hard spheres, have already been investigated elsewhere [6, 7]. They are specific of granular systems, and a consequence of the localized character of the energy dissipation in collisions. In this paper, the volume fluctuations of a vibrated granular gas are studied by means of event driven simulations [8, 9]. The volume of the system changes because the wall on top of the grains is a piston that can move in the vertical direction. The equilibrium position of the piston is determined by equating its weight per unit of area with the pressure of the granular gas just below it. On the other hand, the properties of the fluctuations of the piston around that position, and therefore the volume fluctuations of the system, are not known. If the system were a molecular gas at equilibrium, therefore without being energized by vibration, the total volume fluctuations are known to be Gaussian, and with a second moment that is proportional to the temperature and the isothermal compressibility of the gas [10]. There is no reason to expect the above properties to hold in granular gases, that are inherently in non-equilibrium states. Even more, to give a meaning to them, a first point to be addressed is to specify the notion of temperature to be used. Actually, the equilibrium expression for the volume fluctuations can be used to define a temperature-like quantity, that can be experimentally measured. This is somehow an extension of the usual way of defining “effective temperatures” from the extension to non-equilibrium states of the Fluctuation-Dissipation Theorem. A revision of these ideas in the context of granular media is given in [11]. See also [12] for a discussion of the validity of the Einstein relation in externally driven granular gases. The emerging natural question is whether the effective temperature defined from the relationship between volume fluctuations and compressibility has some other conceptual interpretation, and if it is related to other possible sensible definitions of temperature in the 2