The insertion of metal ions into thiol-derivatized free-base porphyrin monolayers pre-assembled on gold has been conducted by refluxing the metal ion solution in which the monolayer-coated gold electrode was immersed. The extent of the metal insertion was estimated from the decrease in the N1s peak in X-ray photoelectron spectra (XP spectra) assigned to the pyrrole nitrogen which binds a hydrogen atom. The insertion of Co(II) was completed by refluxing for 3 h. Although the extent of the metal insertion for the same reflux time depends on the metal ion used, the insertion of several ions including Mn(II), Fe(II), Ni(II), Cu(II) and Zn(II) was possible. Besides XP spectra, the metal insertion was confirmed by the electrocatalytic activity of the monolayers for the reduction of molecular oxygen. The structural characterization has proved that the monolayer is stable during the reflux;; neither desorption nor change in the orientation of the porphyrin molecules took place. Compared to the commonly used self-assembly of the pre-metalated porphyrins, this post-assembly metal insertion method has an advantage because neither intra nor intermolecular coordinations of the thiol functionality to the central metal ion take place, thus avoiding the unexpected disorder in the monolayer such as the formation of a multilayer, the blocking of the electrocatalytically active central metal ion and the loss of the anchoring functionality or thiol.