There is described the preparation of a flameproofing compound useful in production of cellulosic fibres having a high resistance to combustion, said preparation comprising treating a phosphonitrile halide with a saturated alcohol, treating the halogen containing alkoxy derivative thus obtained with an unsaturated alcohol, and copolymerizing the alkyl substituted alkoxy derivative product thus obtained with an unsaturated acid, forming an alkaline aqueous solution-suspension adapted to be easily and intimately mixed with alkaline aqueous solutions of cellulose xantogenate to obtain a viscous mass of cospinnable components which are co-coagulable in a cellulose xantogenate coagulating bath. Obtaining essentially homogeneous yarns including the flameproofing compound in their cross-sections.