Electron densit ies have been determined /or RF plasmas that were generated within a microwave resonant cavity by measuring the difference of the resonance frequencies with and without plasma. Since that method only yields a value of the electron density weighted ouer the microwave electric field distribution, to obtain real values an assumption on the spatial distribution of the electron density had to he made. Spatial profiles were taken of the emission of a 4s-5p Ar line at 419.8 not (with a small Ar admixture). The electron densities have been determined as a function of pressure and RF power in Ar, CF4, C2 F6 and CHF, plasmas. The results indicate that the electron density for the last three gases decreases as a function of pressure above 50 m Torr. Typical values for the electron density for the investigated parameter range are 1-6 · 103 cm-3. Furthermore, the electron density is the lowest in gases with a high attachment cross .section.