A flame retardant additive includes red phosphorus adsorbed into a porous carrier. The carrier may be mixed with white phosphorus above its melting point (41° C.), so that liquid phosphorus is pulled into the pores of the carrier by capillary action. The phosphorus-loaded carrier may be heated above 250° C. to convert the white phosphorus to red phosphorus. The resulting red phosphorus-loaded carrier may retain flame retardant activity, and may also be protected from the environment for easier handling and formulation. By employing a carrier of a suitably small particle size, it is practical to incorporate the flame retardant red phosphorus-loaded carrier in thin films.