Microstructures and mechanisms of phosphorous incorporation into electrodeposited Ni-P thin films are studied using X-ray diffraction (XRD) and electron probe microanalyzer (EPMA). An atomic pair distribution function calculated from the XRD pattern exhibits the nearest neighbor distance of 2.5 Angstrom and a split double peak at the second neighbor distance that is observed in amorphous structures. The electrodeposited Ni-P alloy is found to have an amorphous structure similar to rapid-quenched Ni-P alloys. The phosphorous concentrations in the Ni-P deposits measured by EPMA decrease with an increase in current densities. The dependence of the phosphorous concentration on the current density is analyzed with respect to the kinetics of chemical reactions in Ni-P electrodeposition. The experimental result agrees well with a solution of rate equations that describe the indirect incorporation of phosphorous. (C) 2003 The Electrochemical Society.