This paper reports the study of the selectivity property of a metalloporphyrins derivative Langmuir-Blodgett thin film, n-octaethyl porphine ruthenium (II) carbonyl (RuOEPCO), to several organic gas samples, namely the saturated vapor of acetone, 2-propanol, ethanol and cyclohexane. The sensing sensitivity was based on the change in the optical property of the film upon exposure to the gas samples that was measured at a particular wavelength, lambda = 514 nm, of the light source. It was observed that the thin film exhibited excellent selectivity property to the gas samples by presenting clear differences amongst its responses. The selectivity property of the thin film was found to depend on the structure of the gaseous molecules of which the highest sensitivity was shown for the gaseous molecules with planar structure, such as cyclohexane. The mechanism for the thin films-gas interaction that promotes the selectivity characteristic is proposed. (c) 2007 Elsevier B.V. All rights reserved.