Using the cluster-variation method in the tetrahedron approximation, we study order-disorder transitions at the (100) surface of Cu-Au alloys. Depending on surface segregation, the order-disorder transition may be first or second order. Disordering of the surface may occur at temperatures above or below the bulk transition temperature. The recent low-energy ion-scattering data on Cu3Au is interpreted as evidence for stronger long-range order at the surface than in the bulk. {\textcopyright} 1984 The American Physical Society.