This paper describes the design, construction and operation of a computerized pH control system for environmental laboratory reactors. Since pH is often a key environmental parameter in environmental process reactors, it is important to have a dependable control system. The design criteria for this system were that it must accurately control pH in 16 different readers with the additional flexibility of allowing each reactor to be controlled and graphed independently at any selected pH. Sampling intervals, mixing intervals, pump-on time, feed volume injection and tightness of pH control (e.g., +/- 0.02 to 0.2) measurement were also possible for each reactor individually. The pH, pump-on time intervals and feed injection volumes were logged into a spreadsheet so that output graphs could be generated automatically for all 16 reactors for up to three years of operation. The supplies, model numbers and costs of main components will be described as well as the detailed instructions on how it was built. Examples are given of operating conditions and output.