The properties of nickel-phosphorus (Ni- P) electrodeposits can be best related to their phosphorus content and microstructure. This study systematically investigated the microstructural evolution and mechanical properties of the deposits plated from nickel sulfamate baths containing 0-40 g dm(-3) phosphorous acid (H3PO3). Experimental results indicate that coarse nickel grains were substantially refined with the incorporation of phosphorus into the deposit. For example, as the deposit phosphorus content was increased from 0 to 14 wt %, the structure of the deposit changed in sequence from a coarse column to a mixture of column and lamella, followed by a well-defined lamella, and finally to a homogeneous amorphous matrix dispersed with nanosized grains. Accompanied with this structural evolution, the deposit exhibited a distinct change in deposit hardness and internal stress. These properties and microstructure relationships are discussed in terms of the lattice defects in the grains and proton discharge during electroplating. © 2005 The Electrochemical Society. All rights reserved.