The molecular orientation, adsorption cross section, and structure of porphin, tetraphenylporphin, copper tetraphenylporphin, and 4-chloromesotetraphenylporphin in benzene solution at a mercury and air interface have been determined experimentally from interfacial and surface-tension measurements. The data fits well to Shereshefsky's equation, and the results are compatible with those obtained from X-ray diffraction measurements on the solid phase.