The organisation and thermal lability of chloro(5,10,15,20-tetraphenylporphyrinato)manganese(III) (Cl- MnTPP) molecules on the Ag(111) surface have been investigated under ultra-high-vaccum conditions, using scanning tunnelling microscopy, low energy electron diffraction and x-ray photoelectron spectroscopy. The Cl-MnTPP molecules are found to self-assemble on Ag(111) surface at roomtemperature, forming an ordered molecular overlayer described by a square unit cell. In accordance with the three-fold symmetry of the Ag(111) surface, three rotationally equivalent domains of the molecular overlayer are observed. The primitive lattice vectors of the Cl-MnTPP overlayer show an azimuthal rotation of ± 15° [esd's???] relative to that of the Ag(111) surface, while the principle molecular axes of the individual molecules are found to be aligned with the substrate −1 10 and − − 121 crystallographic directions. The axial chloride (Cl) ligand is found to be orientated away from the Ag(111) surface, whereby the average plane of the porphyrin macrocycle lies parallel to that of the substrate. When adsorbed on the Ag(111) surface, the Cl-MnTTP molecules display a latent thermal lability resulting in the dissociation of the axial Cl ligand at ~ 423 K. The thermally induced dissociation of the Cl ligand leaves the porphyrin complex otherwise intact, giving rise to the coordinatively unsaturated Mn(III) derivative. Consistent with the surface conformation of the Cl-MnTPP precursor, the resulting (5,10,15,20-tetraphenylporphyrinato)manganese(III) (MnTPP) molecules [if the Cl is now gone it cannot be a molecule but be a cation, or does the surface now function as the negative counter ion?] display the same lattice structure and registry with Ag(111) surface. ; PUBLISHED ; This work was supported by grants from Science Foundation Ireland (P.I. 09/IN.1/B2650