At about 100°C, the effect of magnesium carbonate on the heterogeneous degradation of cellulose is strongly dependent on environmental moisture; the rate of viscosity drop and carbonyl group formation of magnesium-containing samples, appears to decrease with increasing humidity. The net result is that in a dry environment, samples previously soaked in 0.012 NMG (HCO₃)₂ degrade faster than untreated or even de-ashed samples; the opposite is true at about 70% R.H. The rate of degradation of samples treated similarly with calcium bicarbonate is less affected by moisture, and is fairly close to that of untreated samples in dry heating. Correlation of increments in carbonyl groups and viscosity does not show any appreciable influence of the addition of magnesium carbonate on the nature of the process. -- AATA