Scanning tunnelling microscopy (STM) at solid/tetradecane interface is used to study the dynamics of zinc(II)-octaethylporphyrin (ZnOEP) and zinc(II)-meso-tetradodecylporphyrin (ZnTDP) adsorption on the surface of highly oriented pyrolytic graphite (HOPG). ZnOEP exhibits a polymorphic behaviour, with a first metastable phase remaining for the first 2 h. This initial alpha-phase is then converted into beta-phase, where all porphyrins lie parallel to the substrate with a hexagonal arrangement, in agreement with previous reports. At variance with this behaviour, no metastable phases are found during ZnTDP adsorption under the same conditions. We consider that this different behaviour is due to the combination of a stronger interaction and lower mobility of ZnTDP on the HOPG surface. (C) 2013 Elsevier B.V. All rights reserved.