The adsorption behavior of the free base and the zinc(II) complex of cationic meso-tetrakis(N-methyl-4-pyridyl)porphyrins, H2TMPyP4+ and ZnTMPyP4+, were studied at the water\1,2-dichloroethane (DCE) interface by surface second harmonic generation (SSHG) under electrochemical control. The examined porphyrins exhibited specific adsorption features at the water\DCE interface. The surface second harmonic (SH) spectrum and the polarization analysis of SH signals were performed to characterize the interfacial species. Although symmetrically substituted porphyrins show a rather small nonlinear optical response in general, the adsorbed species generated intense SH signals suggesting that the electronic structure of molecules adsorbed at the interface is modified from the bulk state. The surface SH spectrum for H2TMPyP4+ showed similar features to the absorption spectrum in the aqueous phase and the orientation parameter was independent of the bulk concentration. The adsorption equilibrium of H2TMPyP was measured at 295 K, and the Gibbs free energy of adsorption was determined as -29 +/- 1 kJ mol(-1). On the other hand, the surface SH spectrum of the zinc(II) complex system exhibited significant differences from the corresponding bulk absorption spectrum. The surface SH spectrum of ZnTMPyP showed the maximum intensity at 436 nm coinciding with the bulk aqueous species and, furthermore, the additional response centered at 452 nm. The dependence of the SH intensity on the applied potential suggested that the SH response at 452 nm is associated with the J-aggregation of ZnTMPyP monomers adsorbed at the interface.