We have carried out a detailed study of the magnetic switching in square lattice cobalt antidot arrays with periods ranging from 2 μm down to 200 nm (antidot size=antidot separation). Magneto-optical Kerr effect measurements show first a small change in the magnetization due to a reversible rotation of the magnetic spins in the antidot rows, followed by a large change due to reversal of the antidot array columns parallel to the applied field. Employing x-ray photoemission electron microscopy and transmission x-ray microscopy, the latter irreversible process was observed as a nucleation and propagation of discrete domain chains. The propagating chain ends are blocked by perpendicular chains present in the antidot rows via various mechanisms revealed by micromagnetic simulations.