Plasma-chemical reduction of silicon and germanium fluorides with different isotopic composition by hydrogen at low pressure was studied experimentally. Samples of silicon of natural isotopic composition and germanium polycrystalline "flakes" with isotopic numbers 72 and 74 were obtained and used to grow poly- and single crystals by the Czochralski method. The contamination by the most important impurities in each type of silicon and germanium was determined. A chemical mechanism for the reduction process was also proposed. It was shown that the direct method of reduction of isotopically-enriched fluorides is suitable for obtaining small amounts of high-purity isotopes of those elements to be used in unique physical experiments.