A procedure for analyzing the input-output controllability of single-input single-output (SISO) systems is presented. This procedure is applied to a pH neutralization process which must be redesigned to get acceptable controllability. It is found that more or less heuristic design rules given in the literature follow directly. The key steps in the analysis are to consider disturbances and to scale the variables properly. It is suggested that most of the material presented in this paper is suitable for an undergraduate control course.