Capillary electrophoresis was used to study the effect of pH on the mobility of carboxyl-terminated dendrimers ranging from generation two to five. Even though the net charge for G5 (-972) is much higher than that of G2 (-36), its electrophoretic mobility is lower. Titration studies reveal that the effective surface charge density of G5 is lower than the geometric surface charge density, which we attribute to counterion binding. The results suggest a critical condition for nonspecific counterion binding to spheres, analogous but not identical to the well-known Manning condition for counterion condensation on polyelectrolytes.