Modified galactomannans are described having a total molar substitution exceeding 0.7 and containing an average of between 0.7 and 4 hydrophilic substituents and between 0.0001 and 0.02 hydrophobic substituents per anhydroglycosidic unit, the molar ratio of hydrophilic to hydrophobic substituents being between 35:1 and 40,000:1.A process for their preparation is also described.When dissolved in water, the compounds of the invention have the property of modifying the rheological properties of aqueous solutions by increasing their viscosity and their suspending properties, thus making them particularly suitable for industrial use in the paint, paper, and ceramic production sectors and as printing aids in the textile field, as aids in the preparation of fluids used in hydrocarbon research and production, and generally in any application in which such properties are required.