Osmotic pressure measurements were satisfactorily used to compute the effective charge of poly(acrylic) acid (PAA) at different pHs and with different monovalent and divalent counterions. This experimental method is sensitive to the osmotically active species (polymer and counterions). After correction for the polymer contribution to the total osmotic pressure (solvency and excluded volume), the remaining pressure can be attributed to the polyelectrolyte counterions and processed with Donnan and equation of state in order to compute the effective charge number per polymeric chain (Z(eff)). The behavior of Z(eff) against the chain concentration and pH was investigated after neutralization of PAA with LiOH, NaOH, and TMAOH. The results clearly indicate that the nature of the monovalent counterion has no effect on Z(eff) leading to the conclusion that the interaction between monovalent counterions and the acrylate functionality is purely electrostatic in agreement with conductimetric and potentiometric results reported in the literature. The behavior of Z(eff) against the degree of ionization of the polymer and its concentration is also in good agreement with the theoretical expectations of the theory of ionic condensation. Osmotic measurements were also used in order to understand the influence of divalent cations (Mg2+, Ca2+, Ba2+) on Z(eff) of the sodium salt of PAA at pH 9 and at different divalent/acrylate molar ratio. All the divalent cations depress Z(eff) each of one at different degrees, confirming a specific divalent/polymer interaction. The energy of hydration of cations can explain most of the observed results with divalents.