Polysaccharides are polymers frequently introduced into mortar formulations in order to improve the workability and water retention capacity of the fresh materials [1]. Among all the polysaccharides, cellulose ethers seem to be the most suitable molecules to obtain a mortar with pretty good water retention ability (i.e. higher than 94%). On the other hand, polysaccharides such as starch ethers or starches generally induce a low increase in water retention (i.e. up to 85%). Mortar consistency may contribute to its water retention capacity but this hypothesis should be checked by further investigations. Indeed, cellulose ethers could induce excellent water retention thanks to the possible superposition of two phenomena [2]: - a rheological effect similar to the one caused by other polysaccharides; - an effect that could be specific to cellulose ethers, which remains to be defined. It could be caused by a modification of the porous network in the fresh state, osmotic pressure or the presence of a cellulose ether film playing the role of diffusion barrier. The molar mass distribution of polysaccharides has been determined by size exclusion chromatography. Using such well-known molecules, it should be possible to understand the functioning of three types of cellulose ethers (HEC, HPMC and HEMC) on mortar water retention. Then, the influence of mortar consistency on water retention has been studied in order to verify if there is a relationship between these two parameters. To complete the panel, the impact of starch ethers on water retention and mortar consistency has also been investigated.