The present paper describes a flexible thin layer electrochemical flow cell for ultrasensitive amperometric detection at a supported interface between immiscible electrolyte solutions. Nanomolar detection limits were demonstrated using the cell design, and 3D finite element simulations allowed a detailed characterization of the flow cell. The cell design employed in the present work allowed the sensing oil membrane and the aqueous reference electrode to be placed in close contact, thereby minimizing cell resistance. The adjustable cell volume means that the same cell design can be used for different application with different requirement for detection limits and dynamic range. A disposable membrane was employed which reduces the need for surface cleaning and prevents sample carryover between different applications. For the lowest cell volumes the detection chamber approaches a thin layer electrochemical flow cell detector with a large surface to volume ratio.