The complexation characteristics of thorium-arsenazo III in the range of 1-6 mol center dot L-1 hydrochloric acid media were investigated by UV-Vis absorption spectroscopy and computational analysis. The chemical equilibrium model of thorium-arsenazo III complexation was established including the species distribution of arsenazo III, the formation of thorium chloride species, and the release of protons from thorium-arsenazo III complexes. In the spectra of thorium-arsenazo III complexes, two characteristic absorption peaks were observed at 610 and 660 nm, and the latter peak showed a tendency to shift about 4 nm to higher wavelength as the acidity of the hydrochloric acid media increased from 1 to 6 mol center dot L-1. Analysis of the experimental data indicates that the molar absorptivities of both 1:1 and 1:2 complexes (thorium to arsenazo III) steadily increase as the acidity of medium increases. The determined stability constants of 1:1 and 1:2 complexes at various concentrations of hydrochloric acid were extrapolated to zero ionic strength, based on the specific ion interaction theory (SIT) approach. The limiting stability constants were determined to be = 8.56 +/- 0.13 and = 15.17 +/- 0.18 with ion interaction coefficients of Delta epsilon (11) = -0.57 +/- 0.02 kg center dot mol(-1) and Delta epsilon (12) = -0.60 +/- 0.04 kg center dot mol(-1), respectively.