A simple and practical method is presented for the control of first-order flow processes with time varying gain and a pure delay in the measurement of the control variable. It involves on-line identification of the process gain and a subsequent adjustment of the controller parameter. The method is well suited for applications in the chemical process industries where high frequency fluctuations in the process gain are not generally expected. It is tested on a computer simulated process as well as experimentally by application to a continuous stirred-tank neutralization process involving pH control, where buffer concentration in the feed varies with time. There is a potential for the application of this method to an industrial process such as wastewater treatment.