1. A method of recovering vapor emitted when liquid is being dispensed to a tank with the aid of an installation comprising: means (P.sub.L) for dispensing the liquid, designed to deliver said liquid at a liquid flow rate Q.sub.L from a storage tank (100) to said tank, means (113) for measuring said liquid flow rate Q.sub.L, vapor recovery means (P.sub.v ; 126) capable of delivering said vapours at a vapor flow rate Q.sub.v from the tank to a collection tank (100), said vapor flow rate Q.sub.v being controlled by a value g (w; Rx) characteristic of said recovery means, means (123, 122) for measuring said vapor flow rate Q.sub.v, characterised in that said method incorporates the following steps, consisting in: performing an initial calibration of the recovery means (Pv; 126) by air suction by varying said value g, and measuring, for each value g.sup.0.sub.1 of g the corresponding vapor flow rate Q.sub.vi for air in order to build an initial calibration table T.sub.0 : EQU T.sub.0 =[g.sup.0.sub.i,Q.sub.vi ] with each liquid dispensing operation n: measuring the liquid flow rate Q.sub.L at a regular time interval and determining a value g.sup.n-1.sub.1 of the value g to be applied to the recovery means with the aid of the calibration table T.sub.n-1 =[g.sup.n-1.sub.i, Q.sub.vi ] EQU T.sub.n-1 =[g.sup.n-1.sub.j,Q.sub.L ] where Q.sub.vi =Q.sub.L, measuring the vapour flow rate Q.sub.v at each time interval, calculating a coefficient K.sub.n of similarity based on the differences between the measured values of Q.sub.L and Q.sub.v, building a new calibration table T.sub.n to be used for the next dispensing operation n+1 by means of EQU T.sub.n =[g.sup.n.sub.i,Q.sub.vi ]=K.sub.n .multidot.T.sub.0.