The authors have developed a new technique to study by desorption the equilibrium between polyamidic fibre (polyamide 6) and acid dyes; thanks to this method the equilibrium between the desorbing solution and the dyed fibre is obtained with minimum alteration the concentration of the dye in the fibre and pH of the solution. This technique has enabled a deepening in the desorption mechanism, a fixing of the enormous influence of proton in the process and obtaining better values of thermodynamic magnitudes which govern it. The study of these equilibria has been carried out for ßnaphtalenesulphonic acid as a simple pattern of acid «dye» and for the azo dyes C.I., Acid Orange 7 and C.I. Acid Red 141, both monosulphonic, using as desorbing anions C1¯ for the former and ß-naphltalenesulphonate for both. The values of the equilibrium constants have been calculated for several values of the determining variables and at 3 temperatures in every case, with an accordance which we considerer satisfactory. From the values of the constant, the differences of affinity between fixed anion in the fibre and desorbing anion are calculated, the margin of error always being lower than 1 %, finally arriving at the determination of reactive heat with good results.