Scanning tunnelling microscopy was used to investigate the structural formation of methylthiolate self-assembled monolayers on Au(1 1 1). SAMs were prepared by exposing the gold crystal to an ethanol-dimethyldisulfide solution for immersion times of 12 min, 12 h, and 24 h. After preparation the formation of a (4 x root 3) rect. striped phase was found. For this phase, the immersion time is the key parameter determining the size and number of ordered domains. Annealing induced a phase transition leading to large domains in a (3 x 4) structure. The annealing temperature determines whether a mixed phase of both structures or only the (3 x 4) structure are formed. We find no influence of the immersion time on the formation of the second structure. A structure model is presented for both phases on the basis on the same building block containing two methylthiolate molecules and a gold ad-atom. (C) 2010 Elsevier B. V. All rights reserved.