An elemental metal can dissolve at kink sites at low everpotentials producing no new interfacial area. When an ideal solid solution alloy undergoes selective dissolution, this situation is not possible owing to atomic‐scale disorder. Dissolution of the less noble constituent can proceed only by injection of regions of negative curvature into the solid surface, which increases the interfacial area. We present a thermodynamic analysis which accounts for these capillary effects in alloy dissolution. The phenomenon of the critical potential for macroscopic selective dissolution is analyzed in terms of a kinetic roughening transition. This transition results from a competition between curvature‐dependent dissolution and surface diffusion. An expression for the critical potential as a function of alloy composition is developed. The dealloying threshold corresponds to a critical composition on the line of critical potentials defining the roughening transition.