The work describes the in-situ Raman spectroelectrochemical studies on thionine layer covalently bound to the gold surface (Th/Au). The thionine layer was deposited on Au substrate via electrochemical reduction of corresponding diazonium salt. Due to the presence of system of conjugated bonds in the film's structure, grafted thionine reveals electrochemical response analogical to unbound thionine. It is shown that bound Th can undergo irreversible oxidation leading to the formation of surface-bound radical cation and thus thionine dimers, as supported by cyclic voltammetry and Raman spectroelectrochemistry studies. (C) 2017 Elsevier Ltd. All rights reserved.