The electronic properties of pristine and cross-linked (CL) self-assembled monolayers (SAMs) of [1,1';4',1"-terphenyl]-4,4"-dimethanethiol (TPDMT) on Au were studied by electrochemical measurements, including cyclic voltammetry and impedance spectroscopy. In addition, nickel deposition onto the TPDMT and CL-TPDMT substrates was investigated. In all cases, the TPDMT and CL-TPDMT films were found to be insulators, which effectively blocked the ionic permeation of electrolyte, preventing direct access of ions to the Au electrode. At the same time, CL-TPDMT is a better electric insulator than the pristine TPDMT SAM. The top Ni layer in the Ni/CL-TPDMT/Au arrangement was electrically isolated from the Au substrate, and no short circuits occurred. This layer was found to be conductive and relatively stable in the broad potential range in the electrolyte solution.