The aim of this paper is to provide a methodology to study the influence of the molecular weight of polysaccharidic admixtures on mortar properties. A wide range of admixtures with different molecular weight and constant structure was prepared. This has been made possible by the use of ultrasonic treatment, a simple, effective, and without additive method. It appears that cellulose and guar gum derivatives, used as water retention agents, exhibit a significant molecular weight reduction according to the ultrasonic treatment duration. The water retention capacity of the mortars mixed with ultrasonically degraded admixture solutions increases with molecular weight for a same admixture. However, comparing water retention values obtained with similar molecular weight of a same family shows that effectiveness of polysaccharides as water retention agent is not linearly linked to their molecular weight.