Viscosity-modifying admixtures (VMA) are often introduced in the formulation of modern factory-made mortars in order to prevent segregation and to improve the homogeneity and workability of cement-based system. Among VMAs, organic admixtures, and more especially polysaccharides such as cellulose ethers (CE), are widely used, since they improve both rheological property and water retention capacity of the mortars. The present study examines the influence of chemical composition and structure of guar gum derivatives on water retention capacity (WR) and rheological behavior of fresh state Portland-based mortars. The investigation was also completed by adsorption isotherms. For this, original guar gum, HydroxyProplyl Guars (HPG) and hydrophobically modified HPGs were selected. The effect of the molar substitution (MS_HP) and the degree of substitution (DS_AC) was investigated. The results highlight that chemical composition of HPGs has a remarkable effect on fresh state properties of mortars. The original guar gum does not impact both WR and rheological behavior. Increasing MS_HP leads to an improvement of the WR and the stability of mortars while the hydrophobic units further enhance WR and lead to a decrease in the yield stress and an increase in the resistance to the flow of admixed mortars.