[en] A radio frequency (r-f) probe that can provide local void fraction and interface velocity measurements in a gas-liquid two-phase flow was developed. The probe response to bubble passage was investigated with single bubble controlled experiments. For fixed geometry, the probe response was dependent on the dielectric constant of the medium surrounding the probe tip, (air or water), and on the frequency of the carrier signal supplied to the probe. Bubble lengths (< 1 cm) and average bubble approach velocities (< 160 cm/sec) were independently measured by two light sources and detectors placed at a known distance from each other and sensing the passage of each bubble. By choosing a sensitive probe tip length of 2.75 - 3 mm, the r-f probe output provided enough information to determine the bubble length and velocity. The results obtained by the two independent methods show reasonable agreement