A new spectroscopic method was developed to measure in situ the concentrations of dissolved sulfur (IV) species from SO2 absorption in electrolyte systems like seawater and desalination brine. In a recent paper we described UV spectroscopic measurements to fit a thermodynamic model yielding a considerably improved description of the absorption equilibrium. Now, combining this model with a comprehensive spectroscopic calibration, the molar absorptivities of the sulfur (IV) species (SO2(aq), HSO3-, SO32- und SO2Cl-) were determined for a wavelength range from 200 to 350 nm. With the molar absorptivities the concentrations of the sulfur species can be determined straight from a UV spectrum. The validity of the values was cross-checked in different electrolyte systems comparing calculated and measured spectra. The new method yields a very good representation of the species concentrations in SO2 absorption solutions. (C) 2016 Elsevier B.V. All rights reserved.