In the two-fluid model, the closure relations for the mass, momentum and energy interfacial transfer terms involve the contact area between the phases per unit volume, namely the volumetric interfacial area, Ishii suggested that the local volumetric interfacial area should obey a transport equation. The main purpose of this paper is to derive this transport equation from geometrical considerations. No assumption on the interface configuration is needed so that the mathematical expression obtained for the transport velocity is valid for any two-phase flow regime. The physical significance of the transport velocity will be illustrated on some artificially generated bubbly flows with spherical bubbles. The link between the variables entering the transport equation and experimentally measurable quantities will be exemplified. The measurement of the local volumetric interfacial area and its transport velocity can be achieved by using four-sensor probes. As a preliminary study of real measurements, we have assessed the performance of some existing signal processing methods proposed for four-sensor probes on some artificially generated flows.