Small-angle neutron scattering was used to characterize the solution behavior of charged carboxylic acid terminated "cascade" dendrimers (Z-Cascade/methane [[4]/3-oxo-6-oxa-2-azaheptylidyne/3-oxo-2-azaheptylidyne/propanoic acids) of third (G3) and fifth (G5) generations as a function of dendrimer concentration, pH, and ionic strength. An increase in dendrimer concentration leads to a single broad peak in the scattering profile arising from interdendrimer interaction. The dissociation of terminal carboxylate groups also gives rise to an interdendrimer interaction peak, which could be suppressed by the addition of excess salt. The results of contrast matching measurements indicate the accumulation of an excess concentration of tetramethylammonium counterions around the surface of these highly charged particles, and the thickness of these counterions lies somewhere between 4 and 6 A. This conclusion is consistent with our previous potentiometric titration (Zhang, H.; et al. J. Phys. Chem. B 1997, 101, 3494) and capillary electrophoresis (Huang, Q. R.; et al. J. Phys. Chem. B 2000, 104, 898) data.