Electron paramagnetic resonance (EPR) spectroscopy has been used to investigate the structure and binding ability of a novel class of anionic macromolecules : half-generation poly(amidoamine) starburst dendrimers (n.5-SBDs), which differ systematically in size (generation), and which are terminated by sodium carboxylated surfaces. The half-generations in the range 1.5-7.5 have been investigated. To some extent n.5-SBDs mimic both anionic micelles, in their shape and external surface, and biomacromolecules, such as proteins and enzymes, in their internal structure. Positively charged nitroxide radicals attached to carbon chains of different lengths were used to probe hydrophilic and hydrophobic binding sites of the SBD structure. Mobility and polarity parameters were evaluated by means of a computer-aided analysis of the EPR spectra of the interacting radicals. The study of these parameters as a function of pH demonstrated the role of electrostatic interactions in promoting binding at the external SBD/water interface. Such interactions are found to be weaker than those occurring with anionic micelles, due to the greater hydrophilic character of the internal SBD structure. Analysis of the EPR parameters as a function of radical chain length, generation, and carboxylate concentration provided evidence of both hydrophilic and hydrophobic interactions between the probe radicals and n.5-SBDs. The results are consistent with the hypothesis that the radical chain may enter the SBD internal structure and interact with hydrophobic sites. Analysis of EPR spectra at different temperatures provided the activation energies for the rotational motion of the interacting probe.