Abstract: The fifth wheel assembly by which the trailer is demountably coupled to the tractor of a tractor-trailer vehicle is releasably operated by a special decoupling tool which includes a lever bar having a handle end opposite to a fulcrum end with an elongated hook member pivotably mounted therebetween. To unlock the fifth wheel assembly, to allow decoupling of the tractor and trailer, the hook member is looped over the fifth wheel release handle and the fulcrum end of the lever bar is placed in bearing engagement with a suitable side surface of the vehicle so that movement of the lever bar about its fulcrum end will pull the fifth wheel release handle.