The paper addresses control related design issues for neutralization plants. Mainly for control reasons, the neutralization is usually performed in several steps (mixing tanks) with gradual change in the concentration. The aim is to give recommendations for issues like tank sizes and number of tanks. Assuming strong acids and bases, we derive linearized relationships from the disturbance variables (e.g. inlet concentration and flow rate) to the output (outlet concentration), including the scaled disturbance gain, k(d). With local PI or PID control in each tank, we recommend to use identical tanks with total volume V-tot, where we give V-tot as a function of k(d), the time delay in each tank theta, the flow rate q, and the number of tanks n. For k(d) much greater than 1, which is common in pH-neutralization, this gives V-tot = 2qnthetak(d)(1/n). (C) 2003 Elsevier Ltd. All rights reserved.