The characteristics and performance of an autotuning proportional integral derivative (PID) controller developed by us have been investigated. The parameters for this controller, proportional gain, K, the integral time constant, tau(i), and the derivative time constant, tau(d), are on-line-tuned by using heuristic rules, requiring no a priori tuning. The controller showed a stable and robust performance, even when only K was online-tuned. However, the dynamic (or time delay) of the membrane-type dissolved oxygen (DO) probe caused oscillations in the DO concentration unless it was properly compensated. This required a prediction method. When all three controller parameters were on-line-adjusted, the DO probe dynamic could be effectively overcome with no separate prediction step.