Cellulose cotton cloth has been treated with phosphorus, both by direct phosphorylation and by addition of various compounds containing up to 98.6% P. Studies have been made of the effects of these treatments on thermal decomposition and flammability of the cloth. It has been shown that, in terms of unit weight of phosphorus introduced, phosphorylation is a more efficient method of imparting flame retardance than incorporation of additives. When the various additives used were compared on this basis, tritolyl phosphate and triphenyl phosphine were the most effective, while red phosphorus was the least so. Flame retardance of cellulosic materials is generally related to the extent of increased charring. However no such generalization can be made with regard to the phosphorus-based additives studied in this work, since some function primarily in the gas phase and others in the condensed phase. There is thus no obvious correlation between degree of flame retardant effectiveness and mechanism of flame retardance. The predominant effect of nitrogen compounds, both on their own and when used in conjunction with phosphorus compounds, was to decrease flammability, but there appeared to be no simple connection between the magnitude of their effect and any particular mode of action.