The aggregation behavior of the azo dye Acid Red 266 has been investigated in pure water and in the presence of additional electrolyte (0.05 M NaCl) at 20 degreesC by UV/vis spectroscopy and static light scattering (SLS). The spectroscopic data were interpreted as a monomer/dimer equilibrium in the absence and presence of electrolyte. The equilibrium constants were determined as 20 000 +/- 3 000 L mol(-1) tin pure water) and 43 000 +/- 4 000 L mol(-1) (in 0.05 M NaCl), respectively. Although application of SLS revealed much larger species than dimers, the spectroscopic data were evaluated in terms of a monomer/dimer equilibrium to correct the light scattering data, for the first time, by the amount of residual monomers. This is possible because aggregates of this dye, reflecting the same local arrangement between the interacting molecules as in dimers, revealed no differences between dimers and higher multimers in the absorption spectra measured.(1) Thus, the concentration of ah aggregated species is represented by the apparent dimer concentration. The comparison of the corrected mass-based data of the dyestuff aggregates with former uncorrected ones revealed a non-negligible impact of the residual monomer concentration on those quantities. The growth of the aggregates both in length and in cross section has been confirmed by this procedure. Comparison of a former, different spectroscopic analysis with that of the present study gave good agreement in the equilibrium constants for both the salt-free and salt-containing cases.