The porosity of titania and zirconia covered Ag and Au nanoparticles has been investigated using the metal core reactivity as a probe. The presence of pores was confirmed by a newly discovered reaction between halocarbons and core–shell nanoparticles, in which the core gets converted into ions, which are leached out through the shell. Halocarbons having different alkyl chain lengths react with metal cores at different rates due to the differences in the accessibility of the core. It is also observed that the electrochemical accessibility of the core can be reduced by blocking the pores by adsorbates such as cis-dithiocyanato-bis(2,2′-bipyridyl-4,4′-dicarboxylic acid)ruthenium(II) dye (popularly called N3 dye). With the adsorbed dye molecules on the oxide shell, metal cores are stable for extended periods of time even after the addition of halocarbons. The porosity of the Au@SiO₂ system, in which a silica shell is formed over the metal clusters through monolayers, has also been studied. Our studies show that the porosity of different kinds of shells is largely similar, allowing molecular and ion penetration.