This Letter reports the measurement of an upper bound of 0.8 nsec for the time of transition of Sn&sngbnd;SnOx&sngbnd;Sn Josephson junctions from the pair tunneling (V= 0) to the single‐particle tunneling state (V= 2&Dgr;: 1mV at 1.7°K). The threshold is extremely sharp. There is no observed delay in the transition after the threshold is exceeded, and there is no change in the observed risetime of the signal with changes in driving current amplitude or risetime. The magnetic field behavior of these junctions is the same as that observed in the dc case. The combination of high speed, sharp threshold, and strong magnetic field dependence are desirable properties for logic elements.