In this paper, the dielectric constants of colloidal nanocrystals are investigated by scanning capacitance microscopy (SCM) and scanning microwave microscopy (SMM). Whereas SMM provides frequencies from 1 up to 18 GHz, conventional SCM is restricted to values below 20 kHz. With both techniques, C(V) and dC(V)/dV curves are acquired on various nanocrystalline films (Fe3O4, CoFe2O4, PbS) with monolayer thickness and on uncovered reference areas on the same samples. As we find, the dielectric constants of these nanocrystals are significantly larger as those of the bulk materials. A strong decrease of the permittivity with increasing frequency is also found.