Despite of its evident importance, measuring interactivity as a central parameter for characterizing conversations has not experienced much attention so far. Starting with an axiomatic approach and using thermodynamic concepts, this paper proposes the so-called conversational temperature as a new interactivity metric. The resulting scalar parameter is easily to be determined from standard measurements with conversation traces. Several flavors of the method are introduced and investigated for different tasks through a variety of experiments, thus validating the efficiency of our proposal.