The effect of quantum confinement on shallow-donor muonium states and their ionization in nanocrystalline colloidal II–VI semiconductors was studied by the transverse- and longitudinal-field μSR techniques. Evidence is found for the formation of surface muon states on nanocrystalline ZnO with hydrogen passivated surfaces. On nanocrystalline CdSe capped with organic molecules the muons probably form states at the surface or substitute for hydrogen in the organic materials. The results obtained so far on nc-CdTe are suggestive of formation of a muonium state inside the nanocrystals, but further studies are needed to find a definitive proof.