Recent work on the definition of synergistic effects has provided the basis for designing more efficient new approaches to flame-retardant cellulose. The synergistic role of nitrogen in enhancing flame-retardant characteristics has been confirmed for cotton and rayon fabrics modified with N-hydroxymethyl-3-(dialkylphosphono)propionamide (NMPA) and tris(hydroxymethyl)melamine (TMM). Results are compared with those reported previously for similar cellulose derivatives formed via the Arbuzov exchange of halogen precursors. Data are further compared for native and regenerated cellulose derivatives in which NMPA and TMM were applied from a single bath with acid curing, and for others in which TMM was subsequently polymerized by wet fixation in NMPA-modified cellulose. Satisfactory flame-retardant properties can be obtained in cotton at phosphorus contents as low as 0.8%, and the total amount of reagent required is lower as the NMPA/TMM ratio is decreased. An additional contribution of halogen, obtained by using the haloalkyl analog of NMPA, to the flame-retarding effectiveness of these systems is described.