During oxygen bleaching of cellulose, iron and cobalt compounds favour the formation of carbonyl groups along the cellulose chains. Reactions which give rise to carbonyl groups are catalyzed more effectively than those which lead to their destruction. One important factor which contributes to the protective action of magnesium compounds, is a coprecipitation of iron with magnesium hydroxide. An impregnation of the cellulose with a soluble magnesium salt before the introduction of the alkali is therefore advantageous. Other factors contribute to the protective action of magnesium compounds. Experiments with lanthanum salts revealed that these exhibited a behavior very similar to that of magnesium salts. No additive effect of lanthanum plus magnesium was observed. A slight retardation of the degradation of hydro-cellulose was observed during oxygen-alkali treatment in the presence of vanadate. The catalytic effect of copper is masked by EDTA at low hydroxide concentration.