The atomistic mechanisms and dynamics of the (1x1)-->hex surface reconstruction of the clean Pt(100) and its lifting upon CO adsorption have been studied by means of both density functional theory and molecular dynamics simulations based on the modified embedded-atom method. It was found that during the surface reconstruction from the square (1x1) to the similar to20% more dense (quasi)hexagonal phase, the required extra atoms are extracted from the second layer, leaving highly mobile subsurface vacancies, even in the presence of adatom islands or steps. These vacancies will, by surface diffusion, coalesce to form steps. In contrast, during the reverse process-where the hex surface reconstruction is lifted upon adsorption of CO-the roughly 20% excess surface atoms are in a collective manner ejected to form chains of adatoms. In turn, these adatoms coalesce into islands and steps by surface diffusion. A result of the two totally different mechanisms is that the reconstruction process can be concluded not to be the reverse of the deconstruction process. (C) 2004 American Institute of Physics.