In this work we investigate the adsorption characteristics due to exposure to benzene, toluene and chloroform vapor of 2,3,7,8,12,13,17,18-Octaethyl-21H,23H-porphine metal free thin films fabricated by using the Langmuir-Blodgett (LB) thin film technique and its derivatives containing iron chloride, cobalt and magnesium. By using the surface pressure-surface area (II-A) isotherm graphs the optimum conditions for the thin film deposition and mean molecular area values of each porphyrin have been determined. Quartz Crystal Microbalance (QCM) system was employed to investigate the gas sensing performances of thin films during the exposure to Volatile Organic Compounds (VOCs). The surface properties have been investigated by using Atomic Force Microscopy (AFM) and analyzed together with the QCM results to understand the adsorption kinetics of the gas sensing mechanism. The rate constants, k(a) for each thin film interacting with the saturated concentration of vapors have been calculated. The gas sensing interaction has been considered in terms of rate constants in each case. The highest value for k(a) has been observed for benzene exposure. (c) 2012 Elsevier B.V. All rights reserved.