Using a Plate-Plate Rheometer the complex shear, storage and loss moduli (G' and G"), as well as the torque development were monitored under isothermal conditions for several filled and unfilled crosslinkable rubbers. Frequency and strain sweep tests were carried out in order to specify the viscoelastic range for the isothermal vulcanization of the rubber mixes. Although several viscoelastic parameters, like complex and storage shear moduli, the loss tangent (tandelta) and torque as a function of time, reflect the stages of rubber vulcanization, only the tandelta vs time traces followed the progress in vulcanization accordingly. Thus, the curve of the tandelta as a function of time was used to derive the optimum vulcanization time (t(90)) for rubbers. The latter data were compared with those determined by a Monsanto Rheometer. Between the related t(90) values good correlation was found for rubber mixes having t(90) < 15 min (Monsanto), whereas for those showing t(90) > 15 min the proposed method resulted in lower optimum curing times than measured by the traditional Monsanto curemeters.