Cellulose samples were subjected to various degrees of iodination, benzhydrylation and benzhydrylation followed by iodination. The effect of these treatments on crystallinity, rate of thermal degradation, char production and pyrolysis products was investigated. Results indicated that in general the crystallinity index varies inversely with the percent of substitution, rate of weight loss, and amount of residual char. Although the rates of weight loss of the substituted samples increased from 0.3%/min. to as high as 108%/min. depending on the type and percent of substitution, the overall weight loss pattern of the treated cellulose was similar to that of the untreated samples. Of the three types of retardants tried, iodine was the best flame retardant in that the residual char increased by a much larger factor for a given increase in rate. The addition of benzhydryl to the iodinated sample seemed to decrease rather than increase the char producing ability. All three treatments drastically reduce the number of degradation products of molecular weights lower than 150; untreated cellulose gave 59 components whereas the substituted cellulose led to five major compounds; water, acetic acid, furfural, 5 methyl-2-furfuraldehyde, and 1,5 anhydro-2, 3-doxy-beta-D-pent-2-eno-furanose. Of these, water and the furnose derivative were the major components.