The free electron concentration in hydrogen and chemically active plasmas in H-2 + SiF4 and H-2 + GeF4 mixtures was measured by microwave interferometry. The investigations were carried out under conditions of a RF capacitive-coupled discharge at a pressure of 1 Torr. In hydrogen plasma the concentration of free plasma electrons is 1.5 +/- 0.03 x 10(12) cm(-3). When the fluoride is added to the hydrogen plasma, the electron concentration is reduced to 1.1 +/- 0.05 x 10(12) cm(-3) for SiF4 and 9.8 +/- 0.05 x 10(11) cm(-3) for GeF4. It is suggested that the main mechanism responsible for reducing the concentration of free electrons is the mechanism of dissociative attachment of electrons to SiF4 and GeF4 molecules. The difference in the electron concentration for these mixtures is due to the difference in the electron-acceptor ability of the SiF4 and GeF4 molecules determined by the affinity for the electron.