The use of polyol phosphonyl chlorides or phosphochloridates as phosphorylating agents for cellulose is considered as means of introducing a char-forming centre in association with an acid-generating moiety into a functional polymer which itself is potentially char-forming. The specific polyol phosphonyl chloride used, spirocyclic pentaerythritol di(phosphonyl chloride) or diphosphochloridate (SPDPC) is shown to be able to substitute into cellulose (as cotton) in the presence of a suitable solvent (DMF) and base (pyridine and sodium hydroxide) at levels commensurate with phosphorus concentrations up to 2.5% (w/w). This is equivalent to phosphorylation yields of up to 22.7%. The efficiency of phosphorylation is dependent upon reaction temperature and SPDPC: cotton mass ratios. Highest yields occur when reactions are carried out at respective mass ratios of 4.1 (almost equivalent to a molar ratio of an anhydroglucopyranose repeat: 1/2 SPDPC molecule) at 160 degreesC for 2 h. Thermogravimetric analysis of SPDPC-phosphorylated cellulose shows enhanced char formation over the range 400-740 degreesC with respect to untreated cotton cellulose. Scanning electron microscopy shows chars to be well-defined and quite resistant to oxidation as the temperature rises above 600 degreesC in air.