Phosphorus pollution is one of the main factors causing water eutrophication. Phosphorus removal by ferrous salt is one of the common methods for phosphorus removal from wastewater. The effects of Fe(II)/P ratio and pH were investigated and new concepts of "balance reaction" and "unbalance reaction" were proposed to reveal their mechanisms. The results showed that the Fe(II)/P ratio had a significant influence on phosphorus removal by ferrous salt with optimal ratio of 2.25. A relationship between residual phosphorus concentration and total Fe(II)/P ratio was established to predict the dose of ferrous salt. pH also had a significant influence on phosphorus removal by ferrous salt with optimal range of 7.0-8.0, and it was found to be the key factor to regulate the competition between Fe-3(PO4)(2) precipitation and Fe(OH)(2) precipitation. The process of phosphorus removal by ferrous salt was an unbalance reaction. And it was ascribed partly to the requirement for a higher residual Fe(II) concentration to get a lower concentration of residual phosphorus in the solution, and partly to the competition between Fe-3(PO4)(2) precipitation and Fe(OH)(2) precipitation. (C) 2013 Elsevier B.V. All rights reserved.