The chemical reaction equilibrium constant for the formation of pyrosulfite from bisulfite in aqueous solutions {2HSO(3)(-)(aq); S2O52-(aq) + H2O(1)} was determined for temperatures from (313 to 353) K from ATR-IR-spectroscopic and calorimetric investigations of the system (Na2S2O5 + H2O), resulting in ln K = -10.226 + 2123.6/(T/K), Delta(r)Gdegrees(m) = (7.69 +/- 10.33) kJ.mol(-1), Delta(r)Hdegrees(m). = (-17.7 +/- 0.5) kJ.mol(-1) (on the molality scale). Furthermore, some ATR-IR-spectroscopic results on liquid mixtures of (NH3 + SO2 + H2O) at T = 313 K are presented, revealing considerable amounts of pyrosulfite. (C) 2004 Elsevier Ltd. All rights reserved.