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Indigo carmine can be used as model for a blue natural colorant for wool and hair dyeing in a one-bath procedure. The sorption and dyeing properties of indigo carmine on wool are studied as a prerequisite for the successful application in combination with natural dye. To evaluate the suitable conditions for the dyeing process, different pH, dye concentrations and temperatures were studied. The dyeing and sorption characteristics on wool were determined on the basis of spectrophotometry and color measurement. Both high dye sorption and color strength were obtained when dyeing at elevated temperature, e.g. 80°C, and lower pH, e.g. pH 3. However, there is significant potential to apply the dyestuff also at temperatures in the range of 40—60°C and pH values of 4—5. Depending on the dyebath pH the sorption isotherms follow the Langmuir- or Freundlich-type isotherm. At pH 3 and 4 Langmuir isotherms are observed, while at pH 5, 6 and 7 sorption on wool follows the Freundlich isotherm behavior. From the dyeing isotherms the standard affinity (Δµ) of the dyeing step was determined. At pH 3 the heat of dyeing (ΔH) and entropy of dyeing (ΔS) were calculated as 40.22 kJ mol -1 and 0.19 kJ mol-1 K -1, respectively.