The conductance-voltage characteristics of different types of Josephson tunnel junctions have been measured at 4.2 K: symmetric Nb/Al, AlOx/Al/Nb, asymmetric Nb/Al, AlOx/Nb and Nb/Al/AlOx-/AlOx/Nb, containing a double oxidation layer. The symmetric junctions can be described very well by a trapezoidal potential barrier model. The asymmetric junctions show less agreement with theory. In these junctions resistance switching occurs, possibly due to charge trapping. The resistance is lower than in symmetric junctions. The conductance measurements on double oxidation layer junctions show, that this type of junction has an inhomogeneous oxide layer.