In a Wireless Sensor Network (WSN) application for ambient pollution monitoring, which involves the deployment of a large number (i.e. ~100) of sensors in India and UK to obtain fine-grain pollution data, the selection of gas sensors and the validity of sensor measurements against ground truth are vitally important to the success of the system. Electrochemical and solid state gas sensors are low-cost, small sensors that are traditionally used for examining exhaust emissions from vehicles, and hence, they are specifically designed to measure a much higher gas concentration than in our measured environment. In this paper, we evaluate the use of gas sensors in a pollution monitoring project using pervasive sensor technology by literature comparison of the different types of gas sensors available commercially; and a discussion on the calibration process of a set of selected Oxygen (O2), Carbon Dioxide (CO2) and Carbon Monoxide (CO) sensors. The results show that the electrochemical CO sensors exhibit linearity against the range of concentrations that we are interested in (0-20ppm). This paper presents in-depth knowledge on sensor selection and calibration against the ground truth to the research community interested in enabling practical development and deployment of WSN gas pollution monitoring systems. © 2011 IEEE.