Crystalline and amorphous xenon nanoparticles were produced in aluminum by Xe+ ion implantation and were characterized with high-resolution electron microscopy combined with electron energy loss (EEL) spectroscopy. Unusual distributions of the aspect ratio and of the diameter of the crystalline particles were observed and explained, respectively, by minimization of the surface energy and of the strain energy due to the specific lattice mismatch between Al and Xe. Matrix oxidation was revealed as an important phenomenon accompanying the amorphization of Xe particles. Significant variation of relative EEL peak intensities with Xe particle size was observed and associated with unequal pressure inside different particles. The thus revealed variation was utilized to map the pressure distribution inside individual particles with a nanometer spatial resolution.