This paper examines the instability mechanisms of supported liquid membranes in the presence of surface tension gradients. Extraction of nickel with D2EHPA is a typical surface tension negative (sigma(-)) system, while copper extraction with LIX 84 shows a surface tension positive (sigma(+)) system. A sigma(-) system was observed to stimulate the instability of the interface and promote emulsion formation, resulting in more serious loss of membrane liquid from the interface. A sigma(+) system does not produce that effect. The effect of surface tension gradients on the stability of supported liquid membranes is manifested under large gradients, but will be damped out when the aqueous solution contains a high concentration of electrolyte. Smaller membrane pores also decreases the effect. From experiments with different systems, it is deduced that the formation of an oil-in-water emulsion erupting from the interfacial instability is a dominant cause for the instability of supported liquid membranes. The presence of surface tension gradients in the sigma(-) system may supply an additional stimulation for membrane liquid loss. The stability of supported liquid membranes can be improved by preventing or reducing the emulsion formation and large surface tension gradients in the sigma(-) system.