Cellulose flame retarded with an ammonia-cured, polycondensed tetrakis(hydroxymethyl)phosphonium-urea derivative (as Proban CC(R), Rhodia) phosphorylated by cyclic 1,3-propanediol phosphoryl chloride and cyclic 2,2-diethyl-1,3-propanediol phosphoryl chloride can give phosphorus levels up to 6.9% (w/w). Such high levels suggest up to 35.5% yields of reaction if the free secondary amine groups present in the crosslinked flame retardant and the C(6) primary hydroxyl groups are the assumed phosphorylation sites. The presence of substituted propanediol phosphonate moieties in the fibers significantly increases char formation above 400((C))C and scanning electron microscopy indicates that the char has an intumescent structure. The influence of the chemical structure of the propanyl moeity to the reaction extent of the possible phosphorylation sites and the char formation mechanism during thermal pyrolysis of the modified samples are discussed. (C) 2003 Wiley Periodicals, Inc.