The formation of the CO-induced (3 1) structure on Co(1120) has been investigated by scanning tunnelling microscopy (STM). The molecular adsorption of CO is found to create a trough-and-ridge structure running along the [0001] direction and comprising (3 1) periodicity in well-ordered regions. STM measurements conducted during CO exposure show that the adsorption induces migration of Co atoms along the surface. The diffusion of Co species is highly anisotropic, with the preferred direction along [0001], i.e. along the zigzag rows of the clean surface. Migration over distances up to 300 along this direction is observed. A missing row-added row model for the formation of the (3 1) structure is proposed, in which every third zigzag row of atoms in the clean Co(1120) surface is absent. This model can account for the Co migration and the mass balance between removed and added material in the trough-and-ridge structure. The model only partly explains the atomic scale STM images. This discrepancy is attributed to the CO molecules adsorbed on the surface.