<p>When using spectrophotometry for accurate measurement of bisulfate (HSO₄⁻) and sulfate (SO₄²⁻) in a mixed solution, obtaining their respective molar absorption coefficients is a necessity. However, due to the partial ionization of HSO₄⁻ in an aqueous solution, forming a mixture of HSO₄⁻ and SO₄²⁻, it is nearly impossible to directly measure the molar absorption coefficient of HSO₄⁻ through spectroscopy. As a result, there have been almost no reports on the molar absorption coefficient of HSO₄⁻ in aqueous solution. This study utilized UV-Vis spectrophotometry to measure the absorbance of low-concentration solutions of sulfuric acid (H₂SO₄), sodium bisulfate (NaHSO₄), and sodium sulfate (Na₂SO₄) under a nitrogen atmosphere. By combining these absorbance values and the ionization constants of HSO₄⁻ from different solutions, the molar absorption coefficients of HSO₄⁻ at multiple wavelengths were determined by nonlinear fitting. These values were then used to quantify sulfur content in binary mixture solutions of Na₂SO₄/H₂SO₄ and NaHSO₄/H₂SO₄, yielding reliable results. In addition, we implemented time-dependent density functional theory (TD-DFT) to investigate the types of transitions experienced by SO₄²⁻ and HSO₄⁻ at their maximal absorption wavelengths. Concurrently, we conducted an investigation into the molecular origins of the differences in their molar absorption coefficients at the microscopic level. The study’s findings provide valuable scientific and practical insights for fundamental research and industrial monitoring related to HSO₄⁻.</p>