A random replacement IEM model, in which the mean field concentration is considered as a direct averaging over all the existing fluid particles in the stirred tank, is proposed as a numerically more favourable IEM scheme. For steady-state simulations, the present random replacement IEM model is equivalent to the steady-state IEM model. For chemical systems with a stiff kinetics, the random replacement IEM model is found superior to the unsteady-state IEM model. Two numerical examples are given.