Abstract Osmotic pressure measurements on aqueous dispersions of carboxylated latex particles have been performed for different surface charge densities (up to ∼0.48 C m −2) and latex volume fractions. The thermodynamic nonideality exhibited by the solutions was interpreted in terms of counterion binding and it was found that a very substantial fraction of the counterions could be considered bound (in a thermodynamic sense). A behavior reminiscent of counterion condensation was also observed for surface charge densities exceeding ∼0.10 C m −2, i.e., the osmotic pressure became independent of latex surface charge density. The general behavior of the osmotic factor, φ osm, with respect to increasing latex volume fraction, was an initially rapid decrease in φ osm whereupon it leveled off and became almost independent of bulk concentraiton at elevated latex volume fractions. Calculated osmotic pressures based on the promitive model of polyelectrolytes reveal a qualitiative agreement with the present experimental data whereas quantitative agreement is poor.