In situ time-resolved small-angle X-ray scattering is introduced as a method to monitor the diffusion of a solvent in ceramic hollow nanoparticles (HNPs) supported by a polymer gel scaffold. Changes in the form factor were matched to discrete scattering models. A consecutive reaction kinetic model is used to analyze different stages of solvent diffusion. Rate constants and diffusion coefficients are extracted. By taking the diffusion of low molecular poly(ethylene glycol) in poly(ethylene oxide)-embedded HNPs as a model case, it was found that it took about 0.7 s for the solvent to diffuse through the 6 nm thick shell of HNPs and another 1.2 s to fill the inner cavity, while the diffusion coefficient was of the order of 1018 m2 s-1. The results demonstrate that the method can simultaneously measure solvent penetration into the polymer gel and into embedded sub-100 nm HNPs.