The treatment of the effluent produced in the cyanidation of gold and silver ores is one of the main problems that the precious metals industry currently faces. These effluents contain variable amounts of free and complex cyanide (stable and unstable). Different methods for the cyanide elimination have been used; nevertheless, most of the times the consumption of reagents increases operating costs to prohibiting levels. As well, the process may give rise to by-products that are also toxic. Ozone oxidation is a promising alternative for cyanide control that offers several advantages. In this work, the oxidation of cyanide in synthetic alkaline solutions by ozone was studied in a countercurrent bubble column. Dissolved ozone.. cyanide and cyanate profiles were obtained along the column, demonstrating that the cyanide/ozone reaction may take place all along the reactor. This is found to Occur when the mole of O-3/mole of CN rate (specific ozone dose) is higher than one. The oxidation efficiency depends mainly on the specific ozone dose (moles of ozone fed per mole of cyanide), leading to 90% efficiency when the specific ozone dose is 1.2 mol O-3/mol CN or higher. The ozone consumption obtained in these tests (steady-state flow pilot-scale column) was similar to that obtained at the laboratory scale in a semi-batch mechanically stirred reactor (1.2-1.4 mol O-3/mol CN), suggesting that the size of the reactor has no effect on ozone consumption. (c) 2005 Elsevier Ltd. All rights reserved.