The intermolecular dendrimer interactions between poly (amidoamine) dendrimers (SBDs) of different generations (G = 2, 4, and 6) were studied in aqueous solution by EPR and fluorescence depolarization. For the EPR studies a spin probe (TEMPO) was covalently linked to the SBDs, and for the fluorescence depolarization studies a fluorophor (fluorescein) was covalentely linked to the SBDs. The mobility of the TEMPO labeled SBDs was determined by EPR over a range of the concentration of added SBDs (unlabeled SBD of the same generation) from 0.01 to 75% (w/w) in water solutions. The mobility of the TEMPO label was not influenced by added SBD below a concentration of approximately 5%, suggesting that no significant interdendrimer interactions, such as aggregation, occurred. Above a concentration of 30% SBD in water (w/w), the mobility of the TEMPO label decreased dramatically; the effect is mostly attributed to the viscosity increase at such high concentrations. Fluorescence depolarization studies, employing fluorescein-labeled dendrimers are in agreement with the EPR studies.