The purpose of this study was to design and construct an internet-based SCADA (Supervisory Control and Data Acquisition) system for replacing the 10 years old SCADA system made by Philips in air quality monitoring stations around the island. Several important futures, such as digital signal conversions, automatic configurations, web-based human-machine interface, and multi-user data subscriptions were added to create an easy-of-use, robust and reliable system. Several laboratory and field tests were also conducted to evaluate performances of the system in controlled conditions and real world environments. At the end of this study, a list of suggested criteria for the new SCADA system was given to the agency. The internet-based SCADA system has three parts: a signal processing unit based on BX-24 microprocessor and other peripheral chips; an internet protocol processing unit based on SitePlayer chip; and a PC-based web server unit. Together, the system can handle both analog and digital signals from monitoring instruments, store raw data in its nonvolatile memories and transmit data to 8 different remote databases via internet. The user can also control instruments in remote station through web pages. The system passed different tests in our laboratory under simulated conditions, such as repeated power failures, internet service interruptions, and database failures. Field study also indicated that the system could survive under real world environments. The chance of losing data during communication was 4/10000, which means that the system would only spend 1.5 minute to retrieve those data form a 24 hours operation. Results also showed that the system would self-recover in less than 5 seconds when power failure occurred. Two watchdog timers were added to the system to prevent internal system halt. Since then, the system has operated continually without any attention. By using data replication functions provided by MS SQL™, no data were missed during the three-

monitoring station, communication device, internet