Mixed monolayers consisting of octakis(octadecyloxy)phthalocyanine metal complexes (C18PcM; M = Mg, Co, Cu, Zn) and long-chain alkanes on a water surface were investigated by measurements of pi-A isotherms and visible absorption spectra and Brewster angle microscopy observation. C18PcCu and octadecane (C-18) in a 1:7 molar ratio gave a uniform monolayer, where phthalocyanine rings float in parallel with a water surface, and one C18PcCu molecule incorporates seven C-18 molecules to form a molecular complex. It was found that the formation of two-dimensional molecular complexes consisting of C18PcCu and a long-chain alkane shows critical temperature dependence, reflecting the melting point of guest hydrocarbons.