The effects of manganese sulfate monohydrate (MnSO4 H2O, denoted as MSM) on electroless nickel phosphorus (ENP) deposition in acidic bath were systematically investigated with the deposition rate, crystallinity, elemental composition and surface morphology of ENP deposit. MSM was found to act as a stabilizer because the addition of MSM significantly improved the stability of the ENP plating bath. During the stabilizing process, the increase of pH decreased the ENP deposition rate, which is opposite to the conventional concept that higher bath pH gives rise to higher deposition rate. According to these results with the calculated hydrolysis constant of MSM in acid solution, we confirmed that the chemical species to realize stabilizing action was MnOH+. This hypothesis well interpreted the results obtained from the experiments including the bath stability, the MSM and pH effects on the deposition rate and the MSM and pH effects on elemental composition. The proposal of MnOH+. also successfully elaborated the MSM stabilizing actions by the mechanism of anion absorption on nickel catalytic sites. (c) 2014 Elsevier Ltd. All rights reserved.