The presented work is devoted to the evaluation of important validation descriptors concerning the spectrophotometric determination of thorium using Arsenazo III and Chlorophosponazo III. In diluted hydrochloric acid the complex between Th(IV) and Arsenazo III is formed instantly and remains stable for 25 minutes with constant absorbance. Beer’s law is obeyed in the range from 0.70 to 11.64 mol ∙ L-1 . The molar absorptivity at 660 nm is equal to 9.74(±0.09)∙104 cm−1 mol−1 L (data in parentheses stands for standard deviation). The relevant relative standard deviations (RSD) reached ±1.48 %, ±4.34 % and ±3.07, respectively. Thorium concentration can be quantified using Chlorophosphonazo III in media of diluted nitric acid. The linear range of the proposed method was from 1.59 to 18.10 mol L-1 of thorium(IV). The experimentally determined values of molar absorptivity reached ε691 = 3.76(± 0.05)*104 cm-1 mol-1 L. The repeatability and trueness of determination in the linear range did not exceed ±2.2% and 3.3%, respectively. Color stability of the Th(IV)- Chlorophosphonazo III complex maintained a constant value for at least two hours. LOD and LOQ values for both methods were established applying three approaches – the 3-σ, the ULA2 and the Hubaux-Vos ones