Six different equations for the second dissociation reaction constant (K-2) for sulfuric acid were tested using four different Pitzer parameter sets, including two four parameter and two five parameter sets. The best considered electrochemical cell data between 0 and 50 degrees C and water activity data at 25 degrees C were used in fitting the total number of 24 evaluations. Most of the considered equations are able to fit the data within assumed experimental accuracy. A set of four Pitzer parameters with temperature dependency a + b/T is required to present the activity and osmotic coefficient data. The predicted stoichiometric osmotic coefficients at temperatures 0, 25 and 50 degrees C agree well with more complicated models presented in the literature for sulfuric acid-water system. One of the studied K-2 equations was able to extrapolate the stoichiometric osmotic coefficient within 0.01 up to 140 degrees C. (C) 2011 Elsevier B.V. All rights reserved.