Gas-sensing properties of phthalocyanine [(C6H13)3SiOSiPcOGePcOH] Langmuir-Blodgett (L-B) films for halogen gases were investigated. The L-B films were deposited on silicon chips with a built-in microheater and a temperature-sensing diode for rapid control and monitoring of the sensing film temperature. Sensitivities to Cl2, Br2, and I2 at various temperatures were studied. The temperature at peak sensitivity for Cl2 is 130-degrees-C, for Br2 is 60-degrees-C, and for I2 is 20-degrees-C. Sensors with different film thickness were fabricated and tested in Cl2 gas at room temperature. The results show that the conductivity change arises from the both surface and bulk effects. An analytical model is proposed, considering the gas adsorption and desorption on the film surface as rate processes and Fickian diffusion through the film, to describe the behavior of the gas sensor. The model fits the experimental data.