In an apparatus intended especially for connecting electrical conductors by ultrasonic welding, in which a two-part anvil comprising three working surfaces is associated with a sonotrode for defining a compaction chamber on all sides, the two anvil parts are moveable in two different directions relative to each other and the sonotrode and are each driven by a pneumatic cylinder and piston assembly. To achieve an easily handled apparatus, the two pneumatic cylinders are disposed alongside each other axially parallel to each other and to the central axis of the sonotrode in a housing and act upon transmission elements, which deflect the working movements of the pneumatic pistons through a right angle into the anvil parts. As a result the apparatus has a slender external form of small dimensions and enables a pistol grip handle to be fitted, by a trigger of which the functional parts of the apparatus are controlled. In a further embodiment the housing can be releasably connected either to a pistol grip-like handle or to a coupling element.