We present a method for characterizing ultrasmall (<2 nm) supported crystallites with benchtop XRD. Central to the method is an understanding of the intensity effects at play; these intensity effects and their corrections are discussed in depth. Background subtraction�long considered one of the main barriers to ultrasmall crystal characterization�is solved by correcting the diffractogram of a separately measured support for the relevant intensity effects. Rietveld refinement is demonstrated to be an adequate analysis method for the general characterization of simple nanosystems. A 4.8% Pt/SiO 2 sample (1.3 nm, volume-weighted average) is used as a case study; it is found that the Pt spontaneously oxidizes under ambient conditions and consists of a metallic core surrounded by a PtO 2 shell. Both phases have average dimensions smaller than 1 nm. The XRD results also suggest lattice expansion of the Pt core as compared to bulk Pt.