In spite of the unique combination of properties of Re-based alloys and the use of electroless plating in many relevant industries, the early stages of electroless plating of Re-based thin films have not been reported before. This paper is focused on the study of the early stages of growth of ultrathin (<20 nm) Re-Ni films with high Re-content by electroless plating on a functionalized-SiO2 substrate. Each step, from the modification of the substrate to the formation of a continuous film, is characterized. Moreover, the effect of the addition of Ni2+ ions to the bath is analyzed. The addition of a low concentration of Ni2+ ions to the electrolyte is found necessary to obtain full surface coverage and thicker deposits. The as-deposited Re-Ni films consist of both amorphous Ni-Re and H0.57ReO3 phases. The Re-content is not uniform along the thickness of the deposit, and has a maximal value at the percolation point. The deposition process is found to consist of sequential reduction reactions, from ReO4- to lower valence oxides, to metallic Re. (C) 2014 Elsevier B.V. All rights reserved.