The controlled adsorption of molecules of volatile compounds on Langmuir-Schafer films of tetraphenylporphyrins produces a modification of the film optical anisotropy, as revealed by reflectance anisotropy spectroscopy (RAS). These experiments allow a better understanding of the molecular package occurring in the film as well as of its alteration upon the interaction with volatile compounds, helping in the comprehension of the mechanisms responsible for the binding of molecules. The results recommend RAS as a potential transducer technique for chemical sensing. (C) 2009 American Institute of Physics. [DOI: 10.1063/1.3202400]