A mathematical model describing the selective extraction of gold(III) from hydrochloric acid solutions into a membrane consisting of Aliquat 336 chloride immobilized in poly(vinyl chloride) (PVC) is developed. The extraction rate is assumed to be governed by both the rate of chemical reactions occurring at the interface and the diffusion of the Aliquat 336 chloride species within the membrane. The fairly good agreement between the model and experimental results confirms its validity and shows that the model can be used successfully for the mathematical description and optimisation of membrane extraction utilised in analytical and process systems as well as in environmental chemistry.