A mathematical treatment is presented with the aim of describing the time course of the coverage of an electrode surface by a metallic deposit layer or a solid-like organic monolayer when the film undergoes a phase transition, i.e. when the metal layer dissolves or a solid-liquid/gas transition in organic films takes place. An approximate expression is derived for the current which flows through the cell as a response to this process, assuming that the dissolution of the solid phase is initiated at patch boundaries and proceeds as a progressive shrinkage of these patches.