The osmotic and activity coefficients of aqueous sodium and potassium trichloroacetates have been determined at the temperature 298.15 K by the isopiestic method, in the molality range from about 0.2 mol kg(-1) to (9.5 and 8.0)mol kg(-1), respectively. The results have been discussed in terms of ion-ion and ion-water interactions. They confirm our supposition that the main factor which decides the influence of the 1-1 salts on the state of water in solution is the size of the ions.