The effect of jarosite generated from bio-oxidation on gold leaching in acidic thiourea solution was studied, and the role of the oxalate additive was also investigated. The biogenetic jarosite was found to markedly raise the thiourea consumption and hinder the gold dissolution. The dominant cause for the detrimental effect of jarosite is likely that iron ions were released from jarosite dissolution in the acidic solution and thus increased the solution potential, resulting in the accelerated decomposition of thiourea and the aggravated passivation of gold surface by the passivating species from thiourea decomposition. However, the addition of oxalate to solution was shown to be effective in weakening the adverse effect of jarosite manifested as much reduced thiourea consumption and enhanced gold dissolution. The possible acting mechanisms of oxalate were revealed by the analyses of solution potential, Zeta potential and XPS. Oxalate could not only weaken the interaction between Fe³⁺ and thiourea via complexing with Fe³⁺ but also prevent the passivation species from coating on gold surface through indiscriminate adsorption and electrostatic repulsion.