In pH control, the process variable that is very often used is pH itself although use of other alternate variables may provide better performance. In this paper, use of three easily implementable process variables (namely, difference in hydrogen and hydroxyl ion concentration eta, hydrogen ion concentration CH and process pH) are experimentally investigated in the context of two control strategies, namely, PI control and adaptive nonlinear internal model control. Experimental results show that, in general, use of eta provides superior control performance. In the case of adaptive nonlinear internal model control, significantly improved control is obtained with eta as the process variable. This improvement is attributed to the reduction in process static nonlinearity on account, of eta. Besides, ii can also be easily calculated from measured pH for any type of neutralization system. Thus, there is a strong case for using eta instead of pH as the process variable in pH control.