AbstractAlkaline fading reactions of ethyl violet and crystal violet were studied in the presence of polyelectrolytes under high pressure, and the significant role of the hydrophobic hydration effect in polyelectrolyte “catalysis” was demonstrated in terms of the volume of activation (ΔV#). Polyelectrolytes used are poly‐4‐vinylpyridine quaternized with n‐cetylbromide (5 mol‐%) and benzylchloride (95 mol‐%) (C16BzPVP), 3‐6 ionen polymer (polybrene), sodium polyethylenesulfonate (NaPES), and sodium polystyrenesulfonate (NaPSS). These four samples, respectively, enhanced, slightly retarded, retarded and strongly impeded the reactions. The influence of a cationic surfactant, cetyltrimethylammonium bromide, on the rates and the activation parameters was similar to that of C16BzPVP. These experimental findings indicate that the hydrophobic macrocations such as C16BzPVP facilitated the formation of the “ice‐berg” structure of water around the activated complex so that the complex and the macrocations are incorporated in the ice‐berg structure causing decrease in ΔV≠. On the other hand, the hydrophobic macroanions (NaPSS, for example) enhanced the ice‐berg formation around the reactants with resulting increase in ΔV≠. Correspondingly, addition of C16BzPVP decreased the enthalpy and entropy of activation (ΔH≠ and ΔS≠) whereas NaPSS increased both ΔH≠ and ΔS≠. These results can be reasonably accepted if we assume that the contribution of the electrostatic hydration term is much less important than that from the hydrophobic hydration.