The present work deals with the determination of the main bacterial metabolite (sulphuric acid or ferric ions) responsible for nickel release from spent Ni-Cd batteries during bioleaching using the pure culture of Acidithiobacillus ferrooxidans. It was found that Fe(III) ions play the crucial role in Ni bioextraction. They do not work as the oxidation agents; however, they are important in terms of the maintaining of the low pH in the bioleaching media due to their hydrolysis. During the studied leaching processes mainly nickel hydroxides dissolved, however, metallic nickel dissolutions under these conditions did not occur.