Glucose and mannose have a different degree of sweetness, implying different affinity to the sweet taste receptor. While the receptor structure is still undefined, there are several geometrical models for their binding mechanism. A detailed study of the hydration structure of sugars with known degree of sweetness is bound to provide information on the accuracy of such models. Our neutron diffraction study on the hydration of glucose and mannose show that both alpha- and beta-glucose form strong hydrogen bonds with water, and that the steric hindrance of their first hydration shell matches the receptor geometrical model. The alpha-anomer of mannose has a similar, well-defined first hydration shell, but with fewer and weaker hydrogen bonds compared to glucose. Conversely, the hydration shell of beta-mannose (reported as bitter) does not Match the receptor geometrical model. These findings suggest a link between the hydration shell of sugars and their degree of sweetness.