It is known that concentrated solutions of CaCI 2 can cause the breakdown of Portland cement concrete. Recently it has been shown that the severity of CaC12 attack decreases with increasing temperature and above 40°C concrete is not affected. From the above observation it was inferred that the breakdown is due to some compound formation at temperatures below about 20°C. In order to gain a better understanding of the mechanisms of CaCI 2 attack, powders of Portland cement ~both anhydrous and partly hydrated) were shaken in CaCI 2 solutions of various strengths up to 180 days. The temperatures of these suspensions were maintained at 40, 20 and 5°C. The X-ray diffraction and microscopic study of the reacted solids showed that (i) C3A.CaCI2.XH20 forms at all temperatures. (ii) concentrated solution of CaCI 2 leaches out Ca(OH) 2 irrespective of the temperature of storage: (iii) at temperatures below about 20°C complex salts containing CaCI2, Ca(OH) 2 and/or CaCO 3 crystallise out if the CaCI 2 concentration of the mother liquor is 15% or higher. The results indicate that the breakdown of Portland cement concrete, when placed in a concentrated CaCl 2 solution, is not due to the formation of C3A.CaCI2.XH20 or the leaching of Ca(OH) 2 but associated with the formation of complex salts. Subsidiary experiments support the above hypotheses.