The effect of aggregation on the surface pressure, Pi, of monolayers from charged micrometer-sized colloidal particles on the air/water interface is investigated. Pi is completely due to the long-range electrostatic repulsion between the particles mediated by their electrostatic field in the air. The most probable origin of particle aggregation is the attraction between capillary quadrupoles due to undulated contact lines on particle surfaces. Aggregates have higher charge and repel each other stronger than single particles. The data analysis by means of a theoretical model implies that Pi linearly increases with n(1/2); n is the mean aggregation number, which can be determined from the experimental n vs. area curves. The presence of electrolyte promotes aggregation, which tends to increase Pi, but simultaneously reduces the surface charge that leads to lower Pi. For our system, the first effect prevails and apparently paradoxical behavior is observed: the addition of salt in water enhances the electrostatic surface pressure. The data indicate limited aggregation: the rise of the electrostatic barrier prevents the further coalescence of aggregates if they have become sufficiently large. The results contribute for a better understanding of the factors that control the interactions in monolayers of charged particles at liquid interfaces. (C) 2015 Elsevier Inc. All rights reserved.