We consider the applicability of the established rf readout technique, whic
h allows to obtain the supercurrent-phase relation of a Josephson element f
rom impedance measurements in the phase-biased regime. In experiments on Nb
-based single and double tunnel junctions, we demonstrate that this method
holds even if the Josephson coupling energy is smaller than the thermal ene
rgy. Compared with conventional current-voltage measurements, we evaluate t
he rf technique to be favorable in particular for investigations of small s
upercurrents in low-capacitance Josephson elements. (C) 2001 American Insti
tute of Physics.