Process for the location of the position of the interface (5) between two fluids (f1, f2) each contained in a zone (7-1, 7-2) of a reservoir (4) in which: a) a reference transient flow condition that produces a reference pressure difference (P_ref(t)), is initiated in a reservoir (4) that comprises at least one pressure sensor (8) located within one of the zones (Z1), b) later an effective transient flow condition that produces a pressure wave which propagates within the reservoir (4) producing an effective pressure difference (P

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u1?(t)) is initiated, c) the pressure differences (P_ref(t)), (P_u1(t)) are compared, d) a point in time (₁) that represents the instant when the pressure wave reaches the interface (5) between the two fluids (f1, f2) is determined, the distance (L₁) between the interface (5) and the pressure sensor (8) is calculated. The operations b) to d) are repeated in order to monitor movement of the boundary. Application in the petroleum industry.