Process for removing phosphorous from a fiber or yarn

A process for removing phosphorus from a yarn spun from a polymer solution containing polyphosphoric acid, the yarn comprising at least about 1.5 percent by weight of the yarn of phosphorus, comprising contacting the yarn with a base and washing the yarn with an aqueous fluid, wherein the yarn is provided by heating a spun multifilament yarn comprising the polymer and polyphosphoric acid to at least 120 degrees Celsius for a time effective to hydrolyze polyphosphoric acid.