Abstract: The invention relates to a process for monitoring the functioning of a liquid-liquid extraction column in which non-mixing liquids L1 and L2 circulate in counter-current.To this end, an ultrasonic signal is emitted at point Pn of the decanter and this ultrasonic signal is detected at Point P'n located at distance H from point Pn and the time t.sub.n taken by this signal to cover distance H is determined; the speed v.sub.2 of propagation of an ultrasonic signal in the liquid medium near point Pn, preferably constituted by pure liquid L2, is also determined, as well as the speed v.sub.1 of propagation of the same ultrasonic signal in the liquid medium near the point P'n, i.e.