A general method is described for the deposition of metal nanoparticles selectively either inside or outside of carbon nanotubes (CNTs). The method is based on the difference in the interface energies of organic and aqueous solutions with the CNT surface. Because of their lipophilic character, the organic solvent better wets the surface of the nanotubes compared to water and penetrates into the inner volume. The precise control of the volume of each phase allows filling the CNT with the organic phase and covering its outer surface with the aqueous one. Hence, metal nanoparticles can be put with high selectivity either inside or outside the CNT, just by choosing in which solvent the metal precursor is dissolved. SEM, TEM, and 3D-TEM investigations show that a selectivity in localization close to 75% can be reached by this technique. The nanoparticles are homogeneously dispersed and present a narrow size distribution, centered on 5 nm. In this way, one can decorate either the inner or the outer surface of open CNTs, without the need of discriminating the diameter of the opening and without any further step of functionalization than a treatment with nitric acid.