The permeation of gold (III) through a supported liquid membrane impregnated with the phosphine oxide Cyanex 923 in n-decane has been investigated. A model is reported describing the transport mechanism which consists of: diffusion process through the feed aqueous diffusion layer, fast interfacial chemical reaction and diffusion through the membrane. The experimental data are quantitatively explained by mathematical equations describing the rate of transport. Different rate-controlling processes take place as long as the metal transport occurs. High selectivity in the separation of gold (III) with respect to the platinum group metals is achieved.