Mass spectrometry of supported, monolayer-protected, nominally Au-38 (MP-Au-38) clusters is performed via quantitative High Angle Annular Dark Field-Scanning Transmission Electron Microscopy (HAADF-STEM) using size-selected Au-N clusters (N = 25, 38, 55) as mass standards. With the intensity due to the (presumed) 24 hexanethiolate ligands taken into account, the clusters are found to contain 38 +/- 2 Au atoms. The method may also be used to determine the degree of aggregation of deposited nanoparticles.