A new type of self-assembled monolayer (SAM) on gold has been reported. Electroactive SAMs of thionine formed on polyoriented gold and sulphur-modified polyoriented gold from aqueous thionine solutions of pH 7.9 have been characterized by voltammetry. In thionine SAMs formed on gold modified by a monolayer of sulphur adatoms the attachment takes place through S-S bonding between the S adatom and S heteroatom of thionine with edge-on orientation. A fast 2e(-) reduction of thionine molecules in the SAM, at potentials 150 mV more negative than the solution couple, leads to a complete detachment of the reduced molecules leaving the sulphur adlayer intact. In the thionine SAM formed on a flame-treated surface of polyoriented gold, the surface concentration of 4.59 x 10(-10) mol cm(-2) corresponds to the most efficient packing of thionine molecules in a monolayer (end-on orientation). On 2e(-) reduction of thionine molecules, the initial thionine SAM transforms irreversibly to a new surface state with one-half of the initial surface concentration.