Production of intumescent flame-retardant coating compositions which substantially contain a film-forming binder, an ammonium polyphosphate, one or more substances carbonizing under the action of heat, dispersing agents and fillers, if desired. The compositions contain more particularly an ammonium polyphosphate of the general formula:H.sub.(n.sub.-m).sub.+2 (NH.sub.4).sub.m P.sub.n O.sub.3n.sub.+1in which n stands for a whole number averaging a value of more than 400 up to about 800 and the ratio of m:n is about 1.