yComplex formation between a carboxyl-terminated cascade polymer (generation 3) and several cationic polyelectrolytes of varying linear charge density was studied as a function of ionic strength, by turbidimetric titration and dynamic light scattering. Tetramethylammonium chloride was used to adjust the ionic strength in order to avoid sodium counterion binding to dendrimer carboxyl groups. Complex formation occurred abruptly at a critical pH, as signaled by a sudden change in either the turbidity or the apparent Stokes radius from dynamic light scattering. The pH(c) of incipient complex formation was converted to the critical surface charge density sigma(c). Under conditions of low or moderate ionic strength (I), it was confirmed that sigma(c) is roughly proportional to kappa/xi, where kappa similar to I-1/2 is the Debye-Huckel parameter and xi is the linear charge density of the polyelectrolyte.