The first results are reported for a study of the self-assembly of methylene blue (MB), a phenothiazine dye, at sulphur-modified Pt(110), Pt(111) and Pt(100) using cyclic voltammetry and scanning tunneling microscopy (STM). The electrochemical response does not indicate structural differences in the organization of self-assembled monolayers (SAMs) of MB at the three surfaces with different crystallographic orientations. It appears to be dominated by the chemical bond formed between the S adatom and the S heteroatom of MB molecules in the close-packed SAM. MB surface concentrations of 2.57 X 10(-10), 2.17 X 10(-10) and 2.04 X 10(-10) mol cm(-2) were determined for Pt(110), Pt(111) and Pt(100) respectively. STM at sulphur-modified Pt(100) showed perfectly ordered large domains of the underlying Pt(100) and S adlattice, while the MB overlayer appears to be electronically transparent under the imaging conditions used.