The microstructure of electrodeposited Co-P films was investigated using transmission electron microscopy (TEM). The use of ultramicrotomy for sample preparation made it possible to obtain cross sections parallel to the growth direction of the film. Co-P films deposited under constant current conditions are amorphous and no nonuniformity could be identified at a scale of a few to a few tens of nanometers. This is in contradiction with the often made assumption that Co-P electrodeposited under constant current conditions has a columnar microstructure. Co-P layers electrodeposited in pulsed current conditions show a clear multilayer structure. (C) 2004 The Electrochemical Society.