The interaction between sodium dodecyl sulfate (SDS) and blends of polyaniline (PANi) and cellulose acetate (CA) is discussed. The diffusion coefficients of SDS in the blends show that the transport process is accompanied by interactions between the diffusant and the polymeric matrix. In the visible absorption spectrum, the interaction between SDS and CA-PANi blends results in a red shift of the maximum absorption wavelength. This behaviour is explained on the basis of alterations in the lamellar phase of the polymer upon SDS sorption. The interactions between SDS and the blend film are of the Langmuir-type. These blends also show an interesting selective behaviour to SDS with respect to a set of similar surfactants.