A process for the manufacture of bis-methylene spiro-orthocarbonate (BMSOC) wherein methyallyl chloride is subjected to the action of chlorine gas to form a mixture of 1,1-dichloromethyl ethylene and other chlorinated hydrocarbons which need not be and are not separated. The mixture is merely treated wich an alkali metal acetate such as sodium acetate to yield an easily separable second mixture by fractional distillation. The desired 1,1-diacetoxymethyl ethylene is thus recovered in high yield. It is then converted by transesterification in over 90% yield to the bismethylene spiroorthocarbonate (BMSOC). BMSOC is useful as a component for coatings composites, adhesive formulations and the like, which have the unique ability to (a) cure by free radical initiation and (b) expand during cure. BMSOC may be used as a component for dental composites, i.e. filling materials, because it reduces the possibility of leakage at the margin, i.e. at the composite-enamel interface and as a component in bone cements. It is also useful for the reduction of voids in electrical insulating coatings because it reduces the likelihood of corona and/or dielectric breakdown.