Molybdenum was one of the first elements used to retard the flames of cellulosics. More recently it has been used to impart flame resistance and smoke suppression to plastics. Molybdic oxide, ammonium octamolybdate, and zinc molybdate are the most widely used molybdenum flame retardants. Properties are given in Table. These materials are recommended almost exclusively for poly(vinyl chloride), its alloys, and unsaturated polyesters.

Molybdenum trioxide is a condensed-phase flame retardant. Its decomposition products are nonvolatile and tend to increase char yields. Two parts of molybdic oxide added to flexible poly(vinyl chloride) that contains 30 parts of plasticizer have been shown to increase the char yield from 9.9 to 23.5%. Ninety percent of the molybdenum was recovered from the char after the sample was burned. A reaction between the flame retardant and the chlorine to form MoO₂·Cl₂·H₂O, a nonvolatile compound, was assumed. This compound was assumed to promote char formation.

Molybdenum is also a smoke suppressant for poly(vinyl chloride). It promotes the formation of cis- rather than the trans-polymeric decomposition products which are the precursors for smoke. The sources for molybdates are Climax Performance Material Corp. and Sherwin Williams.