A linear stability analysis is applied to a liquid-liquid system in which mass transfer is caused by a reversible interfacial reaction of pseudo-first order of the type present in some metal extraction processes. Numerical analysis of the derived characteristic equation for stationary instabilities indicates that (1) systems are unstable either in the direction of the forward or the reverse reaction; (2) for systems in which both the relevant reactant and the product of the reaction are negatively adsorbed, the unstable direction of transfer is out of the phase in which the diffusivity of the relevant species is lower; (3) the critical disturbance size decreases as the net rate of extraction increases and is independent of the viscosity ratio; and (4) the reactive system is more unstable than a purely diffusional one with no interfacial resistance to transfer. The most unstable case found corresponds to conditions in which the interfacial chemical reaction is not the mass transfer rate controlling step. An explanation for this behaviour is given in terms of the mechanisms of transfer and their rate controlling effect.