Scanning tunnelling microscopy has been used to study the interaction of dimethyl disulphide, (CH 3S) 2, with Cu(111) at room temperature, which is known to produce adsorbed methanethiolate, CH 3S-, by SS bond scission at the surface. The results provide clear evidence for the formation of a pseudo-square surface reconstruction induced by the adsorbate; this reconstruction can be reconciled with the results of previous structural studies of this surface by the techniques of X-ray standing wave and surface extended X-ray absorption fine structure. Two further ordered adsorption phases are observed. One of these, a hexagonal honeycomb phase, is proposed to involve bridge and hollow site adsorption on an unreconstructed Cu surface and is metastable; it is identified as a methanethiolate species previously only studied at low temperature by photoelectron spectroscopy and X-ray standing waves. A third phase of hexagonal symmetry appears to be an alternative endpoint to the pseudo-square phase, but may be due to higher adsorbate coverages; the nature of the structure of the underlying copper surface in this case is unclear.