Dendrimers are nanosized macromolecules with hyperbranched structures that are capable of loading all kinds of active agents for biodelivery. They are monodisperse in terms of size and composition. One of us (YJ) has developed a novel pH-sensitive type of dendrimer, in which the conformation of the polymers is determined by the pH. These dendrimers have a compact, highly convoluted conformation at high pH. At a pH of 6.5 and below, however, the polymers straighten and expand, with the result that any cargo molecules trapped in the coils of the condensed polymers are released. Using SANS with selective deuteration, we will monitor the changes in dendrimer structure brought about by a drop in pH, including pH-dependent changes in particle diameter and density distribution in empty dendrimers, and density changes caused by the release of cargo.