In automated negotiation systems consisting of selfinterested agents, contracts have traditionally been binding, i.e., impossible to breach. Such contracts do not allow the agents to act e ciently upon future events. A leveled commitment protocol allows the agents to decommit from contracts by paying a monetary penalty to the contracting partner. The e ciency of such protocols depends heavily on how the penalties are decided. In this paper, di erent leveled commitment protocols and their parameterizations are empirically compared to each other and to several full commitment protocols. Many di erent aspects of contracting are studied, such as social welfare achieved, CPU-time usage, and amount of contracting and decommitting. If a global clock is used for increasing the decommitment penalties, in nite decommitment loops are prevented, while a local clock cannot guarantee this. Concerning solution quality, the leveled commitment protocols are signi cantly better than the full commitment...