The adsorption of reactive dye mixtures onto chitin modified with 5N KOH was investigated. Three binary mixtures were tested. Each mixture contained Blue D-5RN and either Yellow D-5GN, or Red D-8B or Black DN as the second dye. The tests were conducted without pH adjustment and after pH adjustment to 3.0. The results were based on the constants determined from double Langmuir isotherm. The maximum adsorption capacity of Blue D-5RN (control sample) in the samples without pH adjustment accounted for 97 mg/g d.w., and in the sample with pH adjustment (pH=3) – for 205 mg/g d.w. The adsorption capacity of a mixture of Red D-8B and Blue D-5RN reached 93 and 196 mg/g d.w., respectively, and in a mixture containing Yellow D-5GN – 103 and 214 mg/g d.w., respectively. The dye competition in a mixture was evaluated comparing 7 constants in Langmuir equation for mixtures and Blue D-5RN in a mixture. The adsorption capacity of Blue D-5RN in a mixture was found to be by ca. 8.8 % lower than the adsorption capacity of a dye mixture in the samples without pH adjustment and by ca. 9.4 % lower compared to the samples with adjusted pH. An explicit decline in K₁ value was also observed, depending on the type of the second dye and pH. Blue D-5RN and Red D-8B characterized by a high similarity of chemical structure competed for active sites more strongly. It was confirmed by a higher (ca. 9-fold and 2.1-fold -pH=3) decrease in constants K determined for Blue D-5RN in a mixture. The experimental data showed that in the samples with pH=3, the competition for active sites between dyes was weaker.