We present a study of the different product channels in the reactions of OH and OH-(H2O) with methyl iodide over a range of collision energies. Direct dynamics classical trajectory simulations are employed to obtain an atomistic comparison with the experimental results. For the experiments we have combined a crossed beam ion imaging setup with a multipole rf ion trap. The trap allows us to prepare the molecular and cluster ions with a controlled internal temperature and thus provides well-defined initial conditions for reaction experiments at low collision energy. Changing the internal temperature of the cluster ions was found to have a profound effect on their reactivity.