PRODUCTION OF FUNCTIONAL FLUIDS, PARTICULARLY AIRCRAFT HYDRAULIC FLUIDS, OF IMPROVED FIRE RESISTANCE, COMPRISING A FUNCTIONAL FLUID BASE STOCK, SUCH AS A PHOSPHATE ESTER, E.G., TRI-N-BUTYL PHOSPHATE, OR MIXTURES OF SUCH BASE STOCKS, AND A SMALL AMOUNT OF A SELENOPHENE OR TELLUROPHHENE COMPOUND, PREFERABLY A CHLORINATED SELENOPHENE OR A CHLORINATED TELLUROPHENE, E.G., TETRACHLOROSELENOPHENE OR TETRACHLOROTELLUROPHENE.