The surface of a disordered Pt 2 5 Co 7 5 (100) alloy has been investigated using quantitative LEED, AES and UHV-STM at room temperature. Atomic-resolution images reveal that it reconstructs with close-packed rows shifted by half the interatomic distance, from hollow to bridge sites. The density of shifted rows increases with the surface Pt concentration, leading to (1 x 5), (1 x 6) and (1 x 7) patterns. Segregation and chemical ordering lead to the formation of c(2 x 2) domains between the shifted rows. Chemical resolution was achieved with STM: the apparent height of the Pt atoms in the STM topographs is about 0.1-0.4 a above that of Co, whereas LEED shows that Pt atoms are geometrically ~0.04 a higher. The composition was determined down to the fourth layer. An oscillatory segregation profile is observed, with Pt-rich layers (<C 1 > = 62.6% Pt, <C 1 > = 53.5%) and Pt-depleted layers (<C 2 > = 6.9%, <C 4 > = 2.7%). Chemical ordering is present in the third layer and the four-layers surface slab stabilises with a structure and a composition quite similar to that of the L1 2 PtCo 3 phase. As regards the composition and ordering of the top layer, there is a remarkable agreement between chemically resolved STM analysis and LEED analysis.