The energetic interpretation of liquefaction tests indicates that the behavior of unsaturated sands can be related to the specific energy spent to liquefaction In this paper, an insight is proposed on the role played by the two components of such a specific energy: the specific volumetric energy to liquefaction (Ev,liq) and the specific deviatoric energy to liquefaction (Es,liq). It is argued that Ev,liq (which is nil in saturated soils) can be considered as a state variable to quantify the increase of cyclic resistance caused by a reduced degree of saturation, being thus constant along a given cyclic resistance curve. Es,liq is instead related to a specific value of cyclic resistance ratio or number of cycles at liquefaction via the value of the volumetric component. In fact, for saturated tests (Ev,liq=0) Es,liq is constant for a fixed number of cycles at liquefaction (Nliq). For unsaturated tests, Es,liq increases as Ev,liq increases for a given value of Nliq. The three variables (Ev,liq, Es,liq and Nliq) define a “liquefaction surface” describing the resistance to liquefaction of unsaturated soils.