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Tin dioxide, a king of metal-oxide semiconductor, has a series of important applications such as used in transparent and conducting coatings of glass, 1 in electrochemical modified materials on electrodes, 2,3 and in gas-sensing devices. 4 The requirements for gas sensors include the ability to sense flammable or noxious gases for alarms in homes and factories, to detect the alcohol levels of drivers, and to control the combustion process in industrial furnaces.SnO 2 gas-sensor elements have been prepared from the bulk of sintered powders and from thick or thin films. SnO 2 films have been deposited by a variety of techniques including reactive sputtering, spray pyrolysis, and chemical vapor deposition (CVD). Metallorganic CVD (MOCVD) is an advanced technique 5 to produce thin films. In our previous work, 4 a series of undoped SnO 2 thin films were prepared by MOCVD from Me 4 Sn. These films are very sensitive to CO, H 2 S, and C 2 H 5 OH, demonstrating that thin-film gas sensing elements generally have a fast response to gases and a short recovery period and also have the advantage of economy in production.To detect the freshness of fish is very important in the storage of fish. It is generally detected by calculating the percentage of inosine and hypoxanthine among the adenosine triphosphate (ATP) related compounds in fish muscle. This method takes a lot of trouble and time, because fish samples have to be broken in pieces in order to extract ATP-related compounds. It was reported 6 that gases such as trimethylamine (TMA), dimethylamine (DMA), and ammonia are emitted during the decay of dead fish and that the concentration of TMA increases significantly with the decrease of freshness. So freshness of fish can be determined by detecting the concentration changes of TMA evolved from dead fish. Egashira et al. 7 reported that a sensor element made of sintered SnO 2 powder mixed with Ru was very sensitive to TMA at 550ЊC working temperature and could be used to detect the freshness of fish. To our best knowledge, work on the sensing properties of Th-doped SnO 2 film grown by MOCVD was not reported. This paper presents the results of tests aimed at characterizing the gas-sensing properties to TMA of Th-doped and undoped SnO 2 films deposited by MOCVD from a Me 4 Sn precursor. It was shown that Th-doped SnO 2 film (SnO 2 ϩ Th) was very sensitive to TMA and the response and recovery times were very short. SnO 2 ϩ Th film sensors would be promising for the detection of freshness of dead fish. ExperimentalMaterials.-Tetramethyltin used as the source precursor was synthesized from iodomathine and waterless tin tetrachloride in our laboratory. TMA gas samples of different concentrations were prepared by adding different volumes of TMA to test jars (10 L). Ceramic cylindrical insulating tubes ( 1.2 mm outer diameter, 0.8 mm inner diameter, and 4 mm in length) containing 5 wt % YLiO 2 and 0.5 CaF 2 additive were obtained from Zibo Electronic Appliance Factory. High-purity nitrogen and oxygen were used as the carrier and rea...