The sorption of two anionic azo dyes of similar chemical constitution into PA 6 (polyamide 6) foil was monitored using spectrophotometry and streaming potential measurements. The spectrophotometric data provide a description of the kinetics and the thermodynamics of the dye sorption process and the electrokinetic data provide information about the nature of the interaction. The combination of both methods gives an improved understanding of the dye sorption process.