AbstractUnsaturated spiroorthoesters, which undergo no shrinkage on polymerization, were synthesized in high yield by an elimination reaction of the corresponding halomethylated spiroorthoesters with potassium hydroxide (KOH) or potassiumtert‐butoxide (t‐BuOK) using a phase transfer catalyst. It was found that tetrabutylammonium bromide exhibits a catalytic activity higher than that of the other quaternary ammonium salts, 18‐crown‐6 and poly(ethylene glycol) (M̄w= 300) under equal conditions. The elimination reaction of halomethylated spiroorthoesters proceeds easily in toluene or diglyme with t‐BuOK and intert‐butyl alcohol with KOH leading to the corresponding unsaturated compounds in high yields. In addition, the bromomethylated spiroorthoesters showed higher reactivity than the chloromethylated spiroorthoesters under equa