The composition of natural ferrihydrite varies considerably, especially in terms of aluminum and silicon substitution. This work examines the influence of aluminum content on the redox reactivity of ferrihydrite nanoparticles as determined by kinetic studies of the reductive dissolution of the particles by hydroquinone. Transition state theory applied to variable-temperature experiments is used to measure the activation enthalpy and entropy. The presence of small amounts of aluminum (0-2.1 mol % substitution) causes a decrease in Delta H double dagger and an increase in Delta S double dagger, resulting in an overall increase in reactivity.