We show that selective electroless deposition (ELD) can be used as a tool for electrochemical nanostructuring. In a first step, we bound palladium ions to an aminothiolate (AT) layer on an Au(111) surface. Chemical reduction then served for the fabrication of metallic palladium islands of monatomic height that can in turn activate the ELD of cobalt. We studied the growth of cobalt islands as a function of the oxygen concentration in the deposition bath and of the aminothiolate concentration in mixed amino-/alkanethiolate self-assembled monolayers (SAMs). In situ scanning tunneling microscopy (STM) and X-ray photoelectron spectroscopy (XPS) measurements showed that palladium and cobalt form islands of 1-3 nm up to 60 nm diameter, the size being controlled by the oxygen concentration in the bath.