Electrodeposition of Co-rich Co-Pt alloys from an alkaline amino-nitrite/citrate/glycinate electrolyte is found to depend strongly on the addition of Na-hypophosphite (Na-Hp) in the electrolyte. The presence of Na-Hp shifts alloy deposition to more negative potentials, decreases the partial current for Co deposition, and lowers the Co content in the films. This results in a smaller and more uniform grain size, a better perpendicular orientation of the resulting hexagonal close packed structure, and in the preferential precipitation of oxygen at the grain boundaries. All these microstructural features lead to a higher coercive force, due to the magnetization switching process becoming hindered by the presence of magnetic grain boundaries and possibly by other defects. (C) 2012 The Electrochemical Society. [DOI: 10.1149/2.090204jes] All rights reserved.