Hollow nanoparticles (HNP) have been successfully used for encapsulation and controlled release of biochemicals such as drugs, DNA, and dyes. However no in-situ method has been used to distinguish the chemical composition inside and outside the HNPs, and little is known about the effect of the distribution of particle size, hole size and shell thickness on the process. We intend to use time-resolved SANS to investigate diffusion of water out of SiO2/TiO2 HNP internal voids. In these model experiments we will disperse the HNPs in D2O. Immediately before the experiment, we will replace D2O with H2O surrounding the HNPs. A series of time resolved SANS patterns will then be recorded in order to monitor the slower process of diffusion of D2O out of the HNP voids and its replacement by H2O. Reverse experiments, starting with H2O dispersions, will also be done.