In most cases the pH process is treated as a lumped parameter system for modeling, simulation and control. This approach deals not only with some advantages but also with several limits. After detailed discussion, the distributed parameter model for the pH process is derived for tubular mixers and for the in-line pH process. Although the distributed parameter approach for the pH process is more complicated than classic well-known lumped parameter pH model, it enables several new investigations. Based on the proposed model two research problems are discussed. The first problem deals with the influence of the mixer shape on the pH process dynamics. The second one presents a discussion on the identification possibilities of the reagents concentration in the distributed parameter pH process. The model discussed in the paper was explicated in more detail by the description of imperfect mixing phenomenon. (C) 2011 Elsevier Ltd. All rights reserved.