Zinc oxide (ZnO) is an important semiconducting oxide for gas sensing applications. ZnO sensors have been fabricated in various forms, such as bulk single crystals and sintered pellets [1,2], thick films prepared from an organic binder based paste [3], thin films [4, 5] as well as heterojunctions [6]. ZnObased sensors have been used to detect a wide variety of toxic and combustible gases, such as hydrogen (H2), ammonia (NH9), carbon monoxide (CO), methane and other hydrocarbons [1-6]. Few comparative studies on gas sensing characteristics of ZnO sensors in different physical forms and fabricated via different synthesis routes, are available in the literature [1]. In this paper, we report the comparison of electrical properties and hydrogen gas sensing characteristics of undoped and 1 wt% alumina (A1203) doped sintered ZnO pellets, undoped sintered tape cast ZnO sheets and undoped, chemically deposited thin ZnO films. The undoped and 1 wt % A1203 doped ZnO pellets were prepared from commercially available powders (E. Merck, India) of 99% purity following a conventional ceramic technique and a sintering schedule of 1 h at 900 °C [1, 2]. Final dimensions of the sintered pellets were ~ 0.9 cm diameter and 0.3 cm thickness. The tape cast undoped ZnO sheets were prepared from the same commercial powder as used for the pellets, by the conventional doctor blade technique [7]. The sheets were sintered following the same schedule as that for bulk pellets, but with lower heating and cooling rates of ~ 0.5 °Cmin -1 to avoid warpage at the edges. Sintered dimensions of the square sheets were ~ 1.0 cm × 1.0 cm × 0.08 cm. The undoped ZnO thin films were prepared following a chemical deposition technique [9] from a 0.125 M sodium zincate bath and boiling water. Commercially available microscope glass sides were used as substrates following chromic acid etching and ultrasonic cleaning. A post-deposition air anneal at 150 °C converted the traces of zinc hydroxide [Zn(OH)2] present in the as-deposited film and ensured formation of single-phase ZnO films [10] of ~ 5pro thickness and area 2.4cm × 3.1cm. Phase purity of all samples was checked by an Xray diffraction (XRD) technique. Prior to gas sensing experiments, all samples were palladium (Pd) treated by a proprietory wet chemical technique [8]. The variation of electrical resistance as a function