Comprehensive counterion activity studies have revealed clear influence of the polyelectrolyte concn. and chain length on the polyion-counterion interaction, neither of which is evident from the existing theor. models. The counterion activity coeff. (fa) increases below the overlap concn. c* as calcd. from Odijk's scaling approach. At dilns. where the Debye length (lD) exceeds the contour length (L), this increase is more pronounced. These changes in the ionic interactions are primarily responsible for the strong increase of the equiv. cond. (L) below c*. The findings are not restricted to oligomers but apply to all chain lengths of polyelectrolytes at concns. less than c*. The correlation of fa and L with both c* and the ratio lD/L leads to a general description of polyelectrolyte behavior. Expts. were performed above and below c* and over the range 0.1 ? lD/L ? 10 using a series of poly(vinylbenzyltrialkylammonium) model polyelectrolytes with contour lengths over a range 7 nm ? L ? 102 nm and characterized by a charge d. parameter x = 2.85. [on SciFinder (R)]