A kind of water-soluble binuclear cobalt phthalocyaninesulfonate (bi-CoPc) was synthesized and used to prepare Langmuir-Blodgett (LB) films by the semiamphiphilic technique. On the bi-CoPc-containing subphase, monolayers were built up from the mixtures of octadecylamine (C18NH2) and methyl octadecanoate (OME). The surface pressure-area (pi-A) isotherms indicated that C18NH2-OME molecules could form well-defined condensed-type monolayers. The monolayers of C18NH2:OME = 1:1 and 1:4 were deposited onto solid substrates (CaF2 and silicon substrates). Small-angle X-ray diffraction (XRD) patterns of C18NH2-OME/bi-CoPc LB multilayers indicated the formation of highly ordered multilayers in the vertical direction. The long spacing; of LB multilayers were 5.94 and 5.11 nm for C18NH2:OME = 1:4 and 1:1, respectively. This suggested that in the LB multilayer of C18NH2:OME = 1:4 the hydrocarbon chains stood more vertically than that of 1:1. FTIR spectra confirmed the formation of the supermolecular assembly of C18NH2 and bi-CoPc. W-vis spectra revealed that bi-CoPc existed in aqueous solution as both monomers and aggregates to give a broadened Q-band spectra. The LB films of C18NH2-OME/bi-CoPc produced significantly redshifted Q-band spectra compared with the monomer spectra in solution. These results indicated that bi-CoPc formed dimers in LB multilayers, and the dimers formed between two adjacent layers. From the surface photovoltage spectra (SPS), it could be followed that the photovoltage response of the n-type silicon increased by a factor of about 4 times when modified by the C18NH2-OME/bi-CoPc monolayer. On the contrary, the photovoltage response value of p-Si was reduced by about 30 times compared with that of blank p-Si. These results showed that the C18NH2-OME/bi-CoPc LB monolayer effects different photosensitization processes on n-Si and p-Si.