"The T-1000 was difficult to destroy, since it was seemingly immune to all forms of damage. However, it can be damaged on a molecular level, such as through extreme cold. The T-1000 could apparently endure much heat such as that of a typical diesel fire without damage, but could not withstand being in contact with molten metal.
Once the T-1000 was contaminated with other types of liquid metal it was rendered inoperative. As its material was destroyed in this way, the T-1000 lost control of its functions, changing shapes randomly until enough of its material was destroyed to render it nonfunctional. Since the T-1000 simply reforms instead of truly regenerating itself, the destruction of enough of the molecules that compose the T-1000 eventually led to the demise of the unit." (wikipedia)