The Coulomb blockade of electron tunneling through two tunnel junction
s in series can be suppressed periodically by applying a gate voltage
to the central island. The resulting conductance peaks (''Coulomb osci
llations'') have been studied in aluminum-oxide- aluminum tunnel junct
ions. For decreasing temperature the peaks narrow, in agreement with t
heoretical predictions for a continuous energy spectrum of the island.
We observe an increase of the conductance in the tails of the peaks f
or lower resistance devices, well described by thermally enhanced inel
astic electron co-tunneling.