Abstract: Pyrophoric elemental phosphorus-containing material, e.g., solid waste containing high levels of elemental phosphorus (P.sub.4), is treated by contact with an oxygen-containing gas, such as air, to reduce the amount of elemental phosphorus contained therein while it is otherwise protected from contact with oxygen in the air by being blanketed by a non-flammable fluid (e.g., while under an aqueous protective layer). This renders the material less pyrophoric and also enables the recovery of phosphorus values therefrom, e.g., in the form of a weak phosphoric acid solution. This treatment, for example, mobilizes highly pyrophoric solid waste into a more flowable sludge or mud form while rendering it substantially more non-pyrophoric.