Abstract A new S-induced adsorption phase on Cu(1 1 1) has been observed by scanning tunnelling microscopy. This phase is formed by dissociation of adsorbed methanethiolate in an electron beam, and is found to coexist with the known Cu(1 1 1)/S (√7×√7)R19° and 4 1 −1 4 (`zig-zag') phases. It has a near-square geometry, which we interpret in terms of a significantly expanded pseudo-(1 0 0)c(2×2)-S model. This conclusion is notable in that no equivalent Cu(1 0 0)c(2×2)-S phase exists, and the expanded lattice constant of the new phase is argued to be significant for its stability. Associated with this reconstruction is a strong corrugation which may be due to stress-induced buckling.