Acidic properties of CsCl-yLiCl (y = 0/0.5) and 2CsCl-LiCl + yYCl(3) (y = 0/0.5) melts at 700 degrees C were studied by the potentiometric method using an Pt(O-2)vertical bar ZrO2(Y2O3) membrane oxygen electrode. The acidity of these melts is found to rise together with y. The values of oxobasicity indices (pI(L), or Q for standard solutions) are 3.84 for the CsCl-0.5LiCl melt and 5.55 for the (2CsCl-LiCl)-0.5YCl(3) melt. The main forms of fixation of O2- by the studied melts are Li2O in the CsCl-LiCl system (the stability constant is 7.5.10(4), molar fraction) and YO+ in the CsCl-LiCl-YCl3 system (the stability constant is 1.9.10(4), molar fraction). The solubility of YOCl in the melt of the 2CsCl-LiCl-YCl3 composition is equal to 0.022 (molar fraction). Carbochlorination of the 2CsCl-LiCl-YCl3 melt by the products of CCl4 pyrolysis in argon flow is described by a pseudo-order equal to 1, and the rate constant of this process is (5.4 +/- 0.2) . 10(-3) min(-1) at 700 degrees C. The time required for 10-fold and 100-fold purification from oxide ion traces is (420 and 840) min, respectively.