The observed special behavior of anthraquinone and its derivatives in a Nafion membrane system is reported here. It is observed that anthraquinone (AQ) in aqueous suspension can diffuse through a Nafion membrane if there is a strongly reducing species on the acceptor side of the membrane. In order to explore this phenomenon, the authors use hydroquinone and soluble derivatives of AQ as models. It is shown that the reduction to the hydroquinone form increases the separation coefficient of the species above that expected by its molecular weight. Therefore, AQ must be a special exception as it can only diffuse through the membrane if it is first reduced to the corresponding anthrahydroquinone (AHQ) form. It is shown that a very high surface concentration of AHQ is responsible for the measurable permeation of the membrane by AQ species. The enhancement of membrane transfer by addition of dissolved wood lignin is also explored. (c) 2005 Elsevier B.V. All rights reserved.