A hypothesis has previously been advanced to account for the termination of the dehydrogenation process for the oxidation of South African crocidolite : it was postulated that the presence of magnesium ions would block the electronic migration required for complete oxidation of iron(II) in the temperature range 400° to 500° C. In view of criticism of the validity of the evidence for this hypothesis, new data for the oxidation of Bolivian crocidolite, which has a much higher magnesium content than the South African material, is presented; this is more satisfactory evidence of the validity of the hypothesis.