Assemblies of palladium(II)-5,10,15,20-tetra(4-pyridyl)porphine (tpyp) complex were formed spontaneously at the toluene-water interface, when a tpyp toluene solution was contacted with a PdCl2 aqueous solution under an acidic condition. The interfacial assemblies of tpyp were formed with Pd(II) more effectively than with other divalent metal ions, Ni(II), Cu(II), and Zn(II). It was suggested that Pd(II) of the complex was bound to the nitrogen(s) of the pyridyl groups of tpyp and not to the pyrrole nitrogen. In situ fluorescence microscopy elucidated the formation of two kinds of complex assemblies: assembly 1 (AS1) was generated in a lower tpyp concentration, and assembly 2 (AS2) was generated in a higher tpyp concentration. The fluorescence excitation and emission spectrum of interfacial AS2 showed a red-shift of 7-11 mn in the maximum wavelength relative to that of tpyp in toluene, suggesting a weak pi -stacking interaction between tpyp molecules in the assembly AS2. Time-resolved total internal reflection fluorometry determined the fluorescence lifetime of AS1 and AS2 as 0.15 +/- 0.05 and 1.1 +/- 0.1 ns, respectively. The average stoichiometric composition of Pd/tpyp was suggested to be 3:1 and 1:1 for AS1 and AS2, respectively, by equilibrium analysis, but each tpyp of AS1 was considered to be bound to four Pd(II)-with the fluorescence quenching effect by Pd(II). From these results, we concluded that two Pd(II) were shared between two tpyp molecules and that the Pd(II) bridged the two tpyp molecules of AS1.