A study has been made of the effects of salts, alcohol and surface-active agents, of aontamination by micro-organisms, and of storage at various temperatures on the viscosities of aqueous dispersions of methyl-, methylethyl-, and sodium carboxymethylcellulose. Small concentrations of salts decreased the viscosity of sodium carboxymethylcellulose sols but had no effect on methyl-and methylethylcellulose sols. Larger amounts caused the viscosities of methyl-and methylethylcellulose sols to increase. Industrial methylated spirit, propylene glycol and glycerol increased the viscosities of all dispersions. The effect of sodium lauryl sulphate on sodium carboxymethylcellulose sols was similar to that of salts, and cetrimide precipitated the derivative. Methyl-and methylethylcellulose dispersions increased and then decreased in viscosity with increasing concentrations of sodium lauryl sulphate; cetrimide increased increased the viscosity. All derivatives were degraded by micro-organisms. The viscosity of preserved dispersions of methyl-and methylethylcellulose altered over a year. The greatest decreases occured in the highest viscosity grade samples stored at the highest temperature (30°). All dispersions of sodium carboxymethylcellulose decreased in viscosity over the storage period; storage in light producing the most unstable sols. The pH change of any dispersion on storage was negligible.