Molecular switches and machines that are powered by acid-base reactions are susceptible to low switching cycles because of the concomitant formation of waste products during their operation. Here, we demonstrate the fast, efficient, and reversible modulation of a hydrazone switch in methanol using a visible-light activated photoacid, with no generation of side products, high conversion rates (>95%), and no loss of activity over 100 cycles. TEG functionalization of the hydrazone switch allows for the process to be carried with high conversion (>90%) in water as well.