In water treatment processes, the optimal dosage of coagulant is highly dependent of suspended particle surface charge, size and concentration, pH and composition of water. One way to control the coagulation process can be based on the measurement of the electrophoretic mobility and determination of zeta potential. In this study we investigated the interaction between negatively charged polystyrene latex particles with iron(III) chloride as coagulant. We combined three methods, i.e. dynamic light scattering, nanoparticle tracking analysis, and modelling to thoroughly characterize our system. We have shown that stabilization of zeta potential occurred after 60 to 80 mins after addition of coagulant. We demonstrated different behavior of latex particles with FeCl3 depending on the dosage of iron ions. The optimal dosage of FeCl3 is equal to 1 to 2 mg/L for the rapid aggregation of 10 mg/L latex suspension. We found a good agreement between the aggregation rate and surface charge of the latex particles and that charge neutralization mechanism is responsible for particle aggregation. High dosage of coagulant was also found to result in formation of iron(III) hydroxide particles which diameter was about 200 nm. The initial pH is also important for latex particle coagulation. The lower initial pH of suspension is, the more rapidly the isoelectric point is achieved.