Metallotetraphenylporphyrins (M-TPPs) in DMSO give current peaks at potentials of ca. -700 mV (vs Ag/ AgCl) in cyclic voltammetry (CV) at an Hg electrode. The peaks are non-Faradaic and arise from the formation and destruction of a condensed two-dimensional (2D) phase comprising M-TPPs molecules at the Hg-DMSO interface, which induce an abrupt change in the interfacial capacitance. The monolayer formed by the condensed phase is present on the electrode at potentials more positive than those of the peaks. The condensed 2D phase formed by M-TPPs molecules at the Hg-DMSO interface is homogeneous and stable. Peaks similar to those obtained for M-TPPs on Hg have previously been reported for other, insoluble porphyrin derivatives in H2O, which suggests that this type of compound behaves according to a well-defined pattern in this respect. in recent work, Hosten et al. (Hosten, C. M.; Birke, R. L.; Lombardi, J. R. J. Phys. Chem. 1992, 96, 6585) found meso-tetrakis(4-aminophenyl)porphyrin in acetonitrile to exhibit a peak similar to that for M-TPPs on Hg. They showed the cyclic voltammetric peak for this molecule to arise not from a Faradaic process but from rearrangement of the molecule on the electrode. The potential relationship between this behavior and that observed in M-TPPs is discussed.