The electrodeposition of Ni-P alloy in an emulsified supercritical carbon dioxide (sc-CO2) bath was investigated to explore the effect of H3PO3 concentration (ranging from 0 to 1.6 M) on the material characteristics of the deposits. The experimental results showed that the P content in the deposit increased with increasing H3PO3 concentration in the emulsified sc-CO2 bath, while the weight gain of the Ni-P film decreased. At the same concentration of H3PO3, the significant decreases in weight gain and P content of the Ni-P films deposited in the emulsified sc-CO2 bath were found, as compared with that plated in the conventional aqueous electrolyte at ambient pressure. X-ray photoelectron analyses revealed that the carbon-containing Ni-P deposit could be obtained in the bath with the presence of sc-CO2 fluid. Transmission electron microscopy was employed for microstructure analysis. The average grain size of the Ni-P films deposited in the conventional aqueous electrolyte at ambient pressure or in the emulsified sc-CO2 bath varied, depending on the H3PO3 concentration and P content. The Ni-P films deposited from the emulsified sc-CO2 bath had a higher hardness as compared with that formed in conventional electrolyte. The hardness of Ni-P alloy deposited in the emulsified sc-CO2 bath was enhanced by the additional solid solution strengthening mechanism. (C) 2011 Elsevier Ltd. All rights reserved.