Abstract: An apparatus which is particularly useful in measuring dielectric changes in materials, even when subjected to abrasive and/or high-pressure environments. The apparatus includes a housing adapted to mounting within a vessel containing the material to-be-tested, the housing presenting at least one exposed surface to the interior of the vessel to contact the material and a set of first and second electrodes which are disposed on the exposed surface of the housing to make contact with the material, such that the dielectric properties of the material within the vessel can be monitored. The apparatus is particularly useful in the monitoring of curing of resins in both high and low pressure molding operations, including sheet molding compound (SMC) processes, reactive injection molding (RIM) techniques and similar operations.