We investigate the interactions of generation 3 (G3) poly(amidoamine) (PAMAM) and G3 poly(propylenimine) (PPI) dendrimers with sodium dodecyl sulfate (SDS) in aqueous solution. Size and structure of the dendrimer-SDS aggregates as a function of SDS/dendrimer molar ratio were revealed by SAXS and NMR G3 PAMAM has a relatively open and dense-core structure, while G3 PPI with the same number of surface amine groups possesses a compact and uniform structure. Upon addition of SDS, much more SDS monomers were encapsulated in the interior of PPI rather than in PAMAM. More significant size increase in PAMAM-SDS aggregate is observed at low SDS concentrations, due to the binding of SDS on PAMAM surface and further assembly into larger supramolecular structures. Both noncooperative and cooperative binding of SDS on G3 PPI surface are observed, while only noncooperative binding is proposed on G3 PAMAM, due to its open surface and large surface group distance. The size of the PPI-SDS complex is larger than that of PAMAM-SDS at higher SDS concentrations. Within the investigated SDS concentrations, SDS exhibits much stronger interactions with G3 PPI than with G3 PAMAM. These results provide new insights into dendrimer- surfactant interactions and explain why PPI is much more cytotoxic than PAMAM.