We report on detailed low temperature scanning tunneling spectroscopy measurements performed on preformed nanometer sized gold clusters. The clusters are deposited on Au(1 1 1) films that are coated with a self-assembled monolayer of 1,4-benzenedimethanethiol molecules, thereby creating a double barrier tunnel junction. In this configuration electrons can only move between the scanning tunneling microscope tip and the substrate through tunneling via the cluster. Due to the very small cluster size this results in pronounced single-electron tunneling effects. Anomalies in the I(V) curves measured above a single cluster can be consistently described in terms of charging of the double barrier junction for the majority of the particles. Both Coulomb blockade and Coulomb staircase with step widths in the 200-600 meV range were observed on various individual clusters. The results agree with the commonly used semiclassical theory of charging effects in a double barrier tunnel junction. Furthermore, for a limited number of clusters, additional conductance peaks are observed which are attributed to strong electron confinement inside the nanoscale clusters. (C) 2007 Elsevier B.V. All rights reserved.