Abstract This paper describes an investigation of instability mechanisms of supported liquid membranes (SLMs) in phenol transport system. It was observed that any factor facilitating the formation and stability of oil-in-water (O/W) emulsion, such as high HLB value, low electrolyte concentration and high stirring speed, was favourable to the loss of membrane liquid and the resulting SLMs were unstable. The formation of O/W emulsions accounts for the instability of SLMs in the present system. From a thermodynamics analysis, it is shown that emulsification can be realised with the supply of a suitable energy. The energy in the SLM processes can come from the forced flow of aqueous solutions and the hydrodynamic instability of the interfaces. Once loss of membrane liquid occurs, the vacated space is mainly filled with the membrane phase through the movement along the membrane capillaries and hence further loss can continue in the same way.