The aim of this work has been to study the influence of small changes in the pH of kaolin suspensions on the efficiency of batch coagulation with aluminum reagents. To do that, the efficiencies obtained in the removal of kaolin from suspensions have been interpreted taking into account the values of the concentration of aluminum dosed, pH and z-potential. From the results obtained, it seems clear that the raw pH conditions of the wastewater are important to explain the results of batch coagulation processes because they influence on the speciation and coagulation mechanisms, and thus on the efficiency of the coagulation process. In the particular case studied, the small differences in the initial pH are enough to explain a change in the primary mechanisms from a very cost-effective precipitation-charge-neutralization to a less efficient floc-enmeshment mechanism. Thus, a change of 1 unit of pH in the raw wastewater is enough to decrease the dose of aluminum necessary to obtain similar removal efficiencies (around 85%) from 12 to 2mg Al dm-3. Hence a good knowledge of the system could allow optimizing the performance of operation processes, accepting small doses of reagents in a more robust neutralization charge based process.