Describes a novel device, the 'conductivity probe', consisting of a thin layer of catalyst deposited between two platinum wires wound in a bifilar configuration, developed for observing changes in the electrical conductivity of the catalyst under operating conditions. Using a catalyst of uranium-antimony oxide, it is shown that, when the probe is placed in a reactor also containing the catalyst in its conventional form, measured changes in conductance accompanying adsorption and reaction reliably reflect those taking place in the main bulk of the catalyst.