An on-line corrosion monitoring system, comprising linear polarization resistance, electrochemical noise, and high-resolution electrical resistance probes, has been installed in 3 stainless steel process vessels. Continuous corrosion rate data process information is also being obtained from the plant control system to determine the effect variations in process conditions have on the corrosion in the vessels. This paper describes the background to the system selection, the probes instrumentation and software used and the results from the first few months of operation.