We analyse a model for a continuously stirred tank reactor with imperfect mixing in which the reactor is represented by two well mixed compartments with material transfer between them. These reactors represent 'highly agitated' and 'less agitated' regions. The chemical model used is a quadratic autocatalytic scheme with linear decay of the autocatalyst. We investigate how the reactor performance depends upon the degree of mixing in the reactor and the size of the less agitated region. Surprisingly, the performance of the reactor with sufficiently small values of mixing is inferior to that with no mixing between the compartments. © Austral. Mathematical Soc. 2012.