The paper presents physical properties and kinetics data of sodium sulphite solution, pure or with additives that change surface tension and viscosity. Sokrat44 (copolymer of acrylonitrile and acrylic acid) and short-fiber carboxymethylcellulose (CMC) for the Newtonian and long-fiber CMC for the non-Newtonian viscosity enhancement and Ocenol (cis-9-octadecen-1-ol) or polyethylenglycol (PEG) 1000 for surface tension change were used. The kinetics data are presented under the form of dependencies of oxygen absorption rate (measured in stirred cell) on catalyst (Co2+) concentration, oxygen concentration in gas, pH and temperature of the solution. The method based on Danckwerts' plot for the separate determination of mass transfer coefficient k(L) and gas-liquid interfacial area a in gas-liquid dispersions is described using the kinetics data. Problematic features of the method both at low cobalt catalyst concentrations (deceleration of the chemical oxidation of sulphite in presence of viscosity enhancing additives and non-zero concentration of oxygen in the liquid) and at high catalyst concentrations (shrinking of bubbles in dispersion due to high absorption rates) are taken into account. The method is used in Part II for the determination of k(L) in gas-liquid dispersions produced in mechanically agitated vessel and bubble column. (C) 2004 Elsevier B.V. All rights reserved.