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Gas variations of many mud volcanoes and hot springs distributed along the tectonic sutures in southwestern Taiwan are considered to be sensitive to the earthquake activity. Therefore, a multi-parameter automatic gas station was built on the bank of one of the largest mud-pools at an active fault zone of southwestern Taiwan, for continuous monitoring of CO 2, CH 4, N 2 and H 2 O, the major constituents of its bubbling gases. During the year round monitoring from October 2001 to October 2002, the gas composition, especially, CH 4 and CO 2, of the mud pool showed significant variations. Taking the CO 2/CH 4 ratio as the main indicator, anomalous variations can be recognized from a few days to a few weeks before earthquakes and correlated well with those with a local magnitude> 4.0 and local intensities> 2. It is concluded that the gas composition in the area is sensitive to the local crustal stress/strain and is …