The shelf life of onions, which is one of the major vegetable crops in the world, is reduced mainly due to post-harvest diseases such as sour skin and botrytis neck rot. Recent studies have indicated the difference in the type and concentrations of the volatiles released by the healthy and sour skin infected and botrytis neck rot infected onions. To detect rotten onions in storage, a customized gas sensor array system was developed, which consists of five MOS (Metal Oxide Semiconductor) sensors that are sensitive to wide range of volatiles, electronic and mechanical components, and a specially designed sensor chamber. The chamber is attached with gas delivery system to deliver the volatiles from the onion samples to the chamber. The device is automated using a specially designed electronic circuit which comprises of a microcontroller (16F877A) that controls the functions of every component in the device. The microcontroller also transfers data between the device and PC. The automated electrochemical sensor was exposed to acetone (ketone), acetonitrile (nitrite), ethylacetate (ester) and ethanol (alcohol) to understand the range of each sensor's sensitivity to these compounds. Area under the curve, smellprint and slope features were extracted from the sensor's real time response to different concentrations (1ml, 2 ml and 3ml diluted to 50 ml of water) of four chemicals. Area under the curve and slope values were used to identify the concentration of each chemical and smellprint was used to classify the chemical.