An invasive plate and frame capacitance probe was designed for dielec. const. measurements over a wide frequency range (10-5000 kHz) in a 5-L stirred-tank reactor. Preliminary measurements with polyethylene beads showed a linear variation of the effective permittivity with the vol. fraction of the beads, in accordance with the Maxwell-Wagner theory. Results obtained by bubbling air under various agitation and aeration rates at 500 kHz in deionized water and the effect of a non-ionic antifoam on permittivity variations are shown and discussed; the technique is thought to allow precise local holdup measurements. Existing limitations and possible further improvements of the technique are discussed, esp. in view of the information that can be gained from the std. deviation of the measurements. [on SciFinder (R)]