Abstract A thermodynamic treatment of the various heterogeneity effects on phase transitions at electrified interphases is presented and discussed. It is shown that the properties of a surface transition depend decisively upon the nature of the heterogeneity of the electrode surface. Heterogeneous surfaces with random topography do not favour the occurrence of a phase transition, although it exhibits the same properties as on homogeneous surfaces. Thus it is characterized by the appearance of abrupt vertical steps in the charge density and the surface coverage when they are plotted as functions of the applied potential or the bulk activity of the adsorbate. On heterogeneous surfaces with patchwise topography a phase transition occurs under the same conditions as on homogeneous surfaces. However, the curves of the above plots become continuous without vertical segments as the heterogeneity of the electrode surface increases, i.e. the various plots do not show any sign of the transition but resemble the properties of the stable region of the interphase. Therefore the heterogeneity of the electrode surface generally plays a negative role in both the appearance of a surface phase transition and its detection.