An electrochemical method for the characterization of vulcanization reactions is described. A direct-current voltage is applied to a rubber mixture in a two-part mold which is between the platens of a heating press. The electrical current, e.g. in the nA range, is registered on-line with a sensitive ammeter as a function of the reaction time, The measured curves are characteristic of the vulcanization reaction and can be seen as a kind of "fingerprint" of the rubber mixture. The equipment required for such measurements is described in this publication. The example investigated is the vulcanization of natural rubber with tetraethylthiuram disulfide in the presence of zinc oxide.