A process for making an oxymethylene polymer comprising polymerizing at least one compound capable of forming —CH2O— repeat units (monomer) in the presence of at least one ester of the general formula I (ester): R1—CO—O—R2 (I) wherein R1 can be hydrogen, wherein R1 and R2 are independently of each other linear or branched C1 to C10 alkyl C5 to C7 cycloalkyl —[R3-0-]nR4 wherein R3 is a linear or branched C2 to C5 alkylen and R4 is a linear of branched C1 to C5 alkyl and n is an integer of from 1 to 5. and in the presence of at least one Lewis acid.