The oxidation and subsequent precipitation of iron from solutions are very important for most hydrometallurgical wastewater treatment practitioners. The particle formation process mechanisms are important for shaping the dewaterability and particle size distribution of the precipitates. In this study the objective is to elucidate the mechanisms of iron particles formation from ferrous solutions at high (pH 9.0) and low pH (pH 6.0). The results obtained showed that the precipitation process is dominated by nucleation at the initial stages and aggregation dominated at the final stages. The precipitates formed at higher pH transform faster and result in the formation of more stable precipitates. Precipitates formed at pH 6.0 are larger but less stable chemically. The results may mean higher throughputs and lower costs of post precipitation stabilisation for operations at higher pH. The benefits for operation at lower pH would be a better solid-liquid separation due to the bigger sized particles formed. (C) 2007 Elsevier Ltd. All rights reserved.