We present the results of a synchrotron X-ray scattering study of the thermal disordering of the Au(110)-1 × 2 reconstructed surface. Observing the temperature dependence of the in-plane superlattice and integral-order surface peaks, as well as the (ll0) specular reflectivity, we find that at Tc = 735 K the 1 × 2 surface undergoes a reversible deconstruction transition characterized by a proliferation of compact antiphase defects with no measurable change in the density of surface steps. This transition is described by critical exponents close to those characterizing a two-dimensional Ising transition. We also find that by 784 K there has been a significant increase in the density of surface steps of the type associated with surface roughening. This suggests that the Au(110) surface disorders in a two-step process, deconstruction followed by roughening, with a difference in the two transition temperatures of less than 50 K. © 1991.