The interdependencies of the chemical and physical characteristics of fly ash concrete as they affect sulfate resistance is being studied. Both ASTM Type I and Type II cements have been partially replaced by both ASTM Class F and Class C fly ashes. Cement replacement levels of 25 and 35 volume percent have been studied. Fly ashes have been added as mineral admixtures at the concrete mixing stage and have been interground with the cement constituents as in blended cement production. Concrete specimens have been monitored for linear expansions and mass exhanges while immersed in 10% sodium sulfate solution. This paper reports on the sulfate resistance of fly ash/Type II cement concrete mixtures after 450 days of sulfate exposure. Data from 57 concrete mixtures are included, 28 of which utilized fly ash/cement intergrinding procedures.