The thermodynamic parameters of the formation of the complexes of ethylenediamine N,N' dis uccinic acid (H 4 Y) with Sm 3+ ion are determined by means of calorimetry and measuring pH at 298.15 K and ionic strengths of 0.5, 1.0, and 1.5 (KNO 3 ). The values of , Δ r G, Δ r H, and Δ r S are calculated for the for mation of SmYand SmHY complexes at fixed and zero values of ionic strength. The resulting values are interpreted.