Synopsis The performance in concrete of nine commercial superplasticizing admixtures has been examined by using a particular test procedure to evaluate their performance and to provide comparative data on their effects. All the superplasticizers were observed to have an appreciable fluidifying action in fresh concrete so that, under appropriate conditions, they either considerably improved its workability at normal water content or permitted a water reduction of at least 10% to be made while maintaining normal workability. For very-high-workability concrete deficient in fine material, or with exceptionally high additions of superplasticizer, excessive bleeding and segregation of the fresh concrete occurred, resulting in its rejection. Some superplasticizers also tended to increase the air content of concrete and/or to have a retarding influence upon its setting and hardening characteristics; these effects produced a reduction in compressive strength in some circumstances. For water-reduced superplasticized concretes with air contents below 3%, the compressive strength values obtained at ages from 7 days to 2 years exceeded those to be expected from water reduction alone. Concretes containing super-plasticizers and with appropriate entrained air contents showed a high resistance to freezing-and-thawing actions under wet conditions.