A concept is describe that facilitates the modification of colloidal nanoparticles of different materials with an exactly known number of variable functional groups. In particular, a procedure for the surface functionalization of nanoparticles with a controlled number of mono- or bifunctional poly(ethylene glycol) (PEG) molecules of suitable chain length is reported. Conjugates of nanoparticles with one, two, and three PEG molecules per nanoparticle could be separated using gel electrophoresis. Molecules with additional functionalities could be conjugated to the free ends of the bifunctional PEG molecules. In this way the PEG molecules act as convenient spacers that allow for the sorting of nanoparticles with a discrete number of functional groups.