Abstract: A vibration attenuating device to be used in conjunction with reciprocating-motion type impact mechanisms having working and stop members that are axially movable relative towards or away from one another. The members define a pressure chamber therebetween, which is in communication with a source of pressure medium and is sealed by a flexible sealing material such as an O ring. Each member is acted upon by a compression spring exerting a force on the individual member in a direction towards the other member. The introduction of the pressure medium into the pressure chamber results in a cycle comprising the (1) members being forced away from each other, (2) the pressure medium escaping from the pressure chamber and (3) the members being returned to their original positions by the action of the springs. The invention provides a completely balanced two mass system which is particularly useful in conjunction with impact mechanisms used for hand tools.