The ⁹⁵Mo and ⁹⁷Mo nuclear magnetic resonance study of (Sr,Ba,La)₂(FeMo)O₆ double perovskites is reported. The compounds exhibit a significant low field magnetoresistance at room temperature. Compounds with different (SrBa) - La composition, different content of cation vacancies as well as different Fe - Mo ratio have been studied. The frequency swept spin-echo spectra have been measured at 4.2K and no applied magnetic field. The parent compound, Ba_1.6Sr_0.4FeMoO₆, shows a main line at 59 MHz and a satellite line at 43 MHz. They are attributed to the ideal and defect (antisite neighbours and antiphase boundaries) sites of molybdenum. Both lines correspond to ⁹⁵Mo and ⁹⁷Mo resonances, which are unresolved due to very close gyromagnetic ratios. The spectrum of the 10% La doped sample, Ba_1.44Sr_0.36La_0.2FeMoO₆ shows a higher intensity of the low frequency satellite line, which indicates an increased number of defect sites. The main line is at higher frequencies compared to the parent compound and shows a satellite structure at the high frequency side. An increase of the number of vacancies at the same La doping Ba_1.28Sr_0.32La_0.2FeMoO₆ further increases the population of defect sites. In the Ba_1.28Sr_0.32La_0.2Fe_1.2Mo_0.8O₆ compound the change of the Fe-Mo stechiometry significantly decreases the amount of antisite defects. The results are analysed in terms of the influence of doping on the Mo magnetic moments and population of defect sites. A relation to the magnetic and transport properties of the compounds is discussed.

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