A detailed characterization of electrodeposited Ni-W coatings prepared by pulse electrodeposition on steel and copper substrates is presented. The coatings were obtained at high current pulse frequency and show high microhardness and absence of brittleness. The surface of the coating consists of nanometer sized crystals forming a cauliflower-like structure protected by a mixture of nickel and tungsten oxides. The cauliflower structure is preserved into the bulk coating that exhibits an average composition approximate to 70 at% Ni-30 at% W. Different phases are observed in the bulk structure: a W-rich amorphous phase (approximate to 40%) and Ni-rich crystalline phases (approximate to 60%). The crystalline phases consist of crystalline domains approximate to 7 nm in size of Ni(W) (fcc) solid solution (12 at% W content) and a minor Ni4W component (less than 10%). The amorphous phase exhibits a less compact Ni W structure where some amount of C could also be present. Oxidized W species cannot be detected in the bulk coating, thus discarding the presence of significant amounts of tungsten carbide, tungstates or citrate-tungsten complexes. Our results shed light on controversial points related to the chemical composition and demonstrate the complex structure of this system. (C) 2012 Elsevier Ltd. All rights reserved.