Al&act-An examina tion has been made of the effect of concentration on the temperature dependence of fluidi H,PO,, and the a&h in aqueous solutions of the strong acids HCl, HBr, HNO*, HClO,, I&SO, and 'metal hydroxides LiOH, NaOH and KOH. The results su t that, because Fes of ion association and complex hydration ei%cts in the other systems, only HCI an KOH are capable of giving the least equivocal information about the behaviour of H+ and OH-in fluid motion. It is also concluded that both H+ and OH-are positively hydrated structure-breaking entities, and that despite their special pro . real2 ies their effect on fluidity is not materially different from other positively hydrated structure-b g ions. R&m&-Examen de l'effet de concentration sur la relation entre la fluidite et la temperature dam les solutions aqueuses des acides forts HCl, HBr, HNO,, HCIOI, H&Q et H,FO,, et des hydroxides alcalins LiOH, NaOH et KOH. La r6sultats sugg&rent qu'a cause de l'association ionique et des effets d'hydratation complexes dam les autres syst&nes, HCl et KOH sent seuls susceptibles de foumir les informations les mains &quivoques quant au comportement de H+ et de OH-dans le mouvement du fluide. On a % ement conclu que H+ et OH-sent l'un et l'autre des agents positifs de rupture des structures hy t&s et que, ma&$ leurs propriMs @iales, leurs effets sur la fluidite ne sent pas t&s dilferents de ceux des au&es ions rupteurs de structures hydratees.