The extraction of gold(III) from hydrochloric acid solutions by means of the phosphine oxide Cyanex 925 was studied as a function of different variables: equilibration time, temperature, organic diluent, and concentration of gold, Cyanex 925, HCl, etc.; the system is selective with respect to the extraction of a number of metals. Gold(III) equilibrium loading isotherms were also obtained at 20°C. Experimental data were treated numerically showing that gold extraction can be explained assuming the formation of a complex with the general stoichiometry: HAuCl 4 L 2 , where L represents the extractant. For the extraction reaction log K o e x t = 8.1. The stripping of gold from loaded organic phases can be achieved with water. Data on the extraction of HCl by Cyanex 925 are also presented.