Ultrathin cobalt phthalocyanine films grown on (0 0 0 1) sapphire substrates were investigated for ppb-level gas sensing. For 5 ppb Cl-2, 20 nm films show faster response times (18 s), higher base-line stabilities (<5% drift) and enhanced sensitivity (80%). In contrast, 100 nm films exhibit low sensitivity as well as sluggish response and recovery. Twenty nanometer films owing better crystallinity have high carrier mobility (8 x 10 (4) m(2) V (1) s (1)), and therefore, even ppb level Cl-2 chemisorption leads to a large change in conductance. Cl-2 chemisorption obeys Elovich equation that permits quantitative determination of Cl-2 concentration from response rate of sensor. (C) 2009 Elsevier B.V. All rights reserved.