Many chemical reactions may be accelerated by order(s) of magnitude when exposed to microwaves. Reaction selectivities are often enhanced. Reasons for microwave reaction enhancements are speculative, often conflicting. We have demonstrated that microwaves can change the energies and/or the "effective temperature" of individual species at interfaces. Changes in the relative energies of reacting species or intermediates are shown by Monte Carlo simulation to lead to the observed enhancements in reaction rates or selectivities. Moreover, variations in microwave exposure in time or space can result in significant rate enhancement. Such variations may provide unique rate control.