The molecular orientation and spectroscopic characterization of mixed monolayer assemblies containing free-base mesoporphyrin IX dimethylester and arachidic acid were investigated via the angular dependence of their polarized absorption spectra. The B state absorptions of mesoporphyrins in Langmuir-Blodgett films were split into two red- and blue-shifted components as compared with the B band in solutions. The absorbances of the split bands were enhanced upon increasing the molar ratio of mesoporphyrin in the mixed monolayer assemblies. The transition dipole moments of the two split bands were biaxially oriented with respect to the film plane and the dipping direction. These split bands were interpreted as being due to a strong interaction between the porphyrin B excited states.