Abstract: An on-line monitoring system for 3 trouble gases (H₂, CO and C₂H₄) in transformer oil was designed and fabricated. Teflon AF/ceramic composite membrane was used in the system as a module for separation of the gases from oil, and electrochemical sensors of the RAE systems for H₂, CO and C₂H₄ were used. It was shown that the equilibrium between oil and gas was attained within 4 h by using the composite membrane. A constant temperature enviroment at 30℃ was maintained in determination units by using a thermostat, to eliminate the interference of temperature fluctuations to the response signals of the electrochemical sensors. The temperature control not only could keep the response signals of the electrochemical sensors away from the interference, but also keep longer service life of the electrochemical sensors. The problem of cross-interference between the gases was solved by the matrix calculation of the signal values. As shown by the results given by the monitoring system, the relative errors of determined results of the 3 gases were kept within 15%, and values of RSDs (n=5) found were 7.3% for H₂, 3.9% for CO and 2.1% for C₂H₄.