The commercially available extractant Cyanex((R)) 921 (phosphine oxide) was studied to be applied in the carrier-facilitated transport of gold (1) (cyanide media) and gold (III) (chloride media), across a flat-sheet supported liquid membrane (FSSLM). Gold (I) is transported from alkaline pH values. The presence of lithium salts in the aqueous media improves the transport. In chloride media, the carrier is able to transport gold (III), decreasing the permeability as the initial HCl concentration is increased. From both aqueous media, a model is presented that describes the transport mechanism, consisting of diffusion through a feed side aqueous layer, a fast interfacial chemical reaction, and diffusion of carrier and metal complexes through the organic membrane. The organic membrane diffusional resistance (Delta(o)), aqueous diffusional resistance (Delta(a)) were calculated from the proposed model, and their values were 1.1 x 10(4) and 9.1 x 10(8) s/cm (Delta(o), cyanide and chloride media) and 5.3 x 10(2) and 2.6 x 10(2) s/cm (Delta(a), cyanide and chloride media). The values of the bulk diffusion coefficient (D-o,D-b) and diffusion coefficient (D-o) calculated from the model were D-o,D-b (cyanide) =4.1 x 10(-6) cm(2)/s, D-o,D-b (chloride) = 5.2 x 10(-11) cm(2)/s; D-o (cyanide) =1.1 x 10(-6) cm(2)/s, D-o (chloride) = 1.4 x 10(-11) cm(2)/s. (c) 2004 Elsevier B.V All rights reserved.