An instability of stratified two-phase flow caused by inter-phase mass transfer of a soluble surfactant is the focus of this talk. This theoretical investigation is motivated by recent developments in microscale solvent extraction and two-phase heterogeneous reaction systems. Typically, two phases are brought into contact as parallel flowing streams in a microchannel. Mass transfer of a solute occurs be tween the phases, across a well-defined interface. In cases where the solute is a soluble surfactant, the interfacial tension of the interface will vary with the solute concentration and generate Marangoni stresses that may destabilize the flow. In this study, a closely related model problem is considered in which immiscible fluids flow between flat plates that are held at different solute concentrations. A linear stability analysis, supported by energy budget calculations, reveals the presence of three distinct types of Marangoni instabilities in the creeping flow limit. One of these instabilities is a long wave mode that is caused by interface deformation. The other two are short wave modes that are associated with solutal-convection due to the disturbance flow. When Re is non-zero, inertia introduces the viscosity-indu ced interfacial instability (Yih instability), which interacts with the Marangoni instabilities in a non-trivial manner. The results of this study provide a basis for understanding hitherto unexplained observations of enhanced mass transfer rates in solvent extraction experiments in microchannels.