Oxygen is an essential element in numerous hydrometallurgical leaching processes where it acts as a primary or a secondary oxidant in the dissolution of sulphic concentrates. A common acidic leaching medium is sulphuric acid. It was found crucial to define experimentally the solubility and the rate of mass transfer of oxygen in various sulphuric acid environments prior to direct leaching experiments of sulphuric zinc concentrates, and a laboratory scale equipment was developed for the analysis of oxygen concentrations in aqueous acid media. This paper discusses the solubility and dissolution of oxygen as a function of temperature and concentration of sulphuric acid. Also, the effect of temperature (25-55 degreesC), gas flow rate (0.75-2.25 l/min) and acid concentration (up to 2 M) on the mass transfer coefficient k(L)a between gas and liquid is evaluated. (C) 2002 Elsevier Science Ltd. All rights reserved.