A comparative study has been performed on the sulfate resistance of Portland limestone cement (PLC) mortars exposed to extraordinary high sulfate concentrations (200 g/l). PLCs have been prepared by using two types of clinkers having different C3S/C2S ratios and interstitial phase morphologies. Blended cements have been prepared by replacing 5%, 10%. 20% and 40% of clinker with limestone. Cubic (50 x 50 x 50 mm) and prismatic (25 x 25 x 285 mm) cement mortars were prepared. After two months initial water curing, these samples were exposed to three different sulfate solutions (Na2SO4 at 20 degrees C and 5 degrees C, MgSO4 at 5 degrees C). Solutions were not refreshed and pH values of solutions were monitored during the testing stage. The compressive strength and length changes of samples have been monitored for a period of I year. Additional microstructural analyses have been conducted by XRD and SEM/EDS studies. Results indicated that in general, limestone replacement ratio and low temperature negatively affect the sulfate resistance of cement mortars. Additionally, clinkers of high C3S/C2S ratios with dendritic interstitial phase structure were found to be more prone to sulfate attack in the presence of high amounts of limestone.