The effect of choline addition on the salt-induced super activity of horseradish peroxidase (HRP) is investigated. HRP is presented in the literature as an efficient H2O2 scavenger, and choline is the precursor of glycine betaine, a strong osmoprotectant molecule. Both the regulations of H2O2 and of osmoprotectant concentrations are implicated in plants in order to counteract salt-induced cell damage. For the oxidation of 2,2'-azino-bis(3-ethylbenzothiazoline-6-sulfonic acid) diammonium salt (ABTS), sulfate anions were found to play a crucial role in the increase of HRP activity. This induced super activity can be strongly reduced by adding choline chloride. The phenomena provide an example of physicochemical Hofmeister effects playing a central regulatory role in an important biological system.