The d.c. electrical conductivity of polyacrylate sodium salt and poly(acrylic acid) aqueous solutions of different molecular weights from. 2 to 200 kD has been investigated in a wide range of polymer concentration, covering both the semidilute (unentangled and entangled) and concentrated regimes. The data have been analyzed on the basis of a modified version of the Manning theory of counterion condensation, to cover the semidilute regime, taking into account scaling arguments recently proposed by Dobrynin et al. (Macromolecules 1995, 28, 1859). This analysis, on the basis of the conductivity model we have adopted, provides evidence for a fraction of free counterions that increases with the polymer concentration until the semidilute-concentrated crossover concentration is reached, suggesting that, in the concentration range investigated, the effective charge on the polyion is dependent on the polymer concentration: Moreover, at the higher concentrations investigated, our data suggest that a regime is attained where electrostatic blobs overlap, strongly affecting the electrical transport properties of the solution and causing a progressive, reduction of the free counterion concentration.