The addition reaction of spiro orthoesters (SOEs) with electrophiles accompanying ring-opening isomerization was investigated as a model reaction for polyaddition of bifunctional SOEs with bifunctional electrophiles. Among several electrophiles such as carboxylic acids and carboxylic anhydrides, acid halides showed particularly high reactivities to SOEs. An equimolar reaction of SOEs with acid chlorides took place selectively, leading to the corresponding 1 : 1 adducts. SOEs with seven-membered cyclic ether rings-1,4,6-trioxaspiro[5.6]undecane derivatives-showed higher reactivities than SOEs with six- and five-membered cyclic ether rings. The reaction accompanied zero shrinkage in volume.