The KLL Auger electron spectra of Zr (Z = 40) and Nb (Z = 41) following respectively the electron capture (EC) decay of 90Nb and the de-excitation of the isomeric state of 91mNb were investigated in detail using a combined electrostatic electron spectrometer and radioactive sources prepared by surface sorption on a polycrystalline carbon substrate. For both elements, energies, relative intensities, and natural widths of all the nine well-resolved basic spectrum components were determined and compared with theoretical predictions and with results of previous measurement as well. Results of our ab initio multiconfiguration Dirac-Hartree-Fock calculations are discussed also in relation to the solid-state effect. Indications of an influence of the EC decay on the absolute energy of the KL2L3(1D2) transition (so-called atomic structure effect) in Zr were found. The effect of the retarded current-current interaction on the KL1L2(3P0) transition intensity was observed to be appreciable for the investigated elements in accordance with the prediction.