A large number of data on mobility and mass have been newly obtained or reanalyzed for clusters of a diversity of materials, with the aim of determining the relation between electrical mobility (Z) and mass diameter dm = (6m/pi p)1/3 (m is the particle mass and p the bulk density of the material forming the cluster) for nanoparticles with dm ranging from 1 nm to 6.5 nm. The clusters were generated by electrospraying solutions of ionic liquids, tetra-alkyl ammonium salts, cyclodextrin, bradykinin, etc., in acetonitrile, ethanol, water, or formamide. Their electrical mobilities Z in air were measured directly by a differential mobility analyzer (DMA) of high resolution. Their masses m were determined either directly via mass spectrometry, or assigned indirectly by first distinguishing singly (z = 1) and doubly (z = 2) charged clusters, and then identifying monomers, dimers, . . . n-mers, etc., from their ordering in the mobility spectrum. Provided that dm > 1.3 nm, data of the form dm vs. [z(1+mg/m)1/2/Z)]1/2 fall in a single curve for nanodrops of ionic liquids (ILs) for which p is known (mg is the mass of the molecules of suspending gas). Using an effective particle diameter dp = dm+ dg and a gas molecule diameter dg = 0.300 nm, this curve is also in excellent agreement with the Stokes-Millikan law for spheres. Particles of solid materials fit similarly well the same Stokes-Millikan law when their (unknown) bulk density is assigned appropriately.