The absorption spectra of three 2,6 substituted 1,5 dihydroxy 4,8 diamino 9,10 anthraquinone dyes in water organic mixed solvent were measured at different acidities. The spectral changes were explained. The pK a values corresponding to the different ionisation step were determined. The model molecular structure of the title compounds were calculated by AM1 method. The results are consistent with the experimental phenomenon.
