Experimental measurement of rf electric fields in electron‐beam/plasma interactions requires a non‐perturbing probing technique. Such a technique is described in this paper and has been applied to study cathodic electron beam break‐up and meniscus formation phenomena observed in a low‐pressure mercury‐vapor discharge. Experiments are described in which the deflection of a fine electron‐beam probe directed parallel to the cathode surface is measured to determine the spatial variations of the low‐ and high‐frequency electric fields. A number of supporting experiments are reported which help to clarify the various phenomena occurring in the beam/plasma interaction region.