The performance of a continuous well mixed isothermal polymerisation reactor under periodic variation of inlet monomer concentration is theoretically analysed for average degree of polymerisation (<(DP)over bar>) increase. Two values of residence time produce polymer with identical degrees of polymerisation under conventional steady-state operation Under periodic operation the average degree of polymerisation is significantly different for these two values of the reactor residence times. The larger value of the residence time gives a higher average degree of polymerisation.