Speeding reactions through ionic strength Brønsted acidity is introduced into microporous zeolites through the addition of framework aluminum. Pfriem et al. show that in the presence of water, the limited volume in the microchannels of zeolite H-MFI leads to a high concentration of hydrated hydronium ions at aluminum sites. The resulting high charge density creates a highly non-ideal solvation environment and, for cyclohexanol dehydrogenation, the charged carbenium-ion transition state was stabilized. A higher rate was maintained with lower-acidity sodium ion–exchanged zeolites that had the same high ionic strength. Science , abh3418, this issue p. 952