The findings of this study which evaluated 21 portland cements identified K2O and Na2Oeq as significantly contributing to ASR expansion with a less strong correlation to SO3 content The conclusions are based on results of ASTM C 441 Tests. Response surface modeling has identified a negative trend of clinker SiO2 and a direct trend of Na2Oeq as significantly contributing to ASR expansion. The ASTM C 227 analytical results were less conclusive and proved to be a poor indicator of the effects of cement variables in the expansion in tests up to 1 year in duration. In contrast, ASTM C 441 rapidly and clearly delineated the cements contribution to ASR expansion.