The exceptional toughness of the double network (DN) hydrogels originally developed in the group of J. P. Gong is normally caused by the energy dissipated by chain breakage in the tightly crosslinked first network. The loosely crosslinked second network holds the material together and ensures a large damage zone forms around a crack tip. We have studied the load-extension (or compression) properties and, in particular, the load-unload hysteresis of a number of DN gels to examine this chain breakage and look for other energy dissipation mechanisms. When chain scission was the dominant energy loss mechanism non-recoverable hysteresis was observed, that is to say the reloading occurred up the unloading curve. The unloading curve was fitted to a Langevin-style rubber elasticity model, giving the density and average length of the first network chains at the current maximum extension. The chain breakage, which occurred when the extension was increased beyond the previous maximum, caused a decrease in the chain density and an increase in the average chain length. In one system the two networks contained base and acid groups respectively and, at an appropriate pH, recoverable hysteresis was observed, presumably caused by the breakage of the acid-base interactions on loading.