Mg amalgam reacts with PTFE (foil or oriented film on Si) to give a thin surface layer containing MgF2 in a mixture with complicated, air-sensitive, carbonaceous product containing large amount of residual C-F bonds. The reaction does not propagate into the bulk polymer, which, consequently, retains its white color even after hundreds of hours of reaction at 150 degreesC. These findings contrast with the reactivity of PTFE with amalgams of alkali metals, Li, Na, K. The differences are interpreted, in terms of the electrochemical model of amalgam carbonization, as blocking of charge (e(-)/Mg2+) propagation through the modified layer. AFM patterns show that the Mg-treatment increases the surface roughness. The molecular-level ordering of PTFE films is strongly perturbed by the action of Mg-amalgam; the treated surface shows only small proportion of organized macromolecules.