To obtain 1,1,2,2-tetramethylpropyl peroxy ester useful as a polymerization initiator for vinyl monomers or a curing agent for unsaturated polyester resins.

This 1,1,2,2-tetramethylpropyl peroxy ester is represented by formula I {(n) is 1 or 2; R is a cycloalkyl, an unsubstituted or a methyl- substituted phenyl or CH(R1)R2 [R1 is H or a 1-4C alkyl; R2 is H or a 1-10C alkyl] when (n) is 1 and R is a cycloalkylene, phenylene or CH(R1)R2CH(R3) [R1 and R3 are each H or a 1-4C alkyl; R2 is a 0-10C alkylene] when (n) is 2} which is the objective compound. The compound is obtained by reacting an acid chloride represented by formula II (R' is same as R) with 1,1,2,2- tetramethylpropyl hydroperoxide in the presence of an alkaline compound of tertiary amine below the decomposition temperature of the compound. When the compound represented by formula I is used as a plymerization initiator for vinyl monomers, the residual monomers and oligomers in the resultant polymer are reduced.