This work concerns a comparison of the hydration properties of two simple molecules of biological relevance: trimethyl-amine-N-oxide (TMAO) and tert-butyl alcohol (TBA). These molecules are geometrically very similar, having the same hydrophobic moiety and different polar groups. Both molecules have been used as a model to study hydrophobic behaviour in water solutions. In the present work, dielectric relaxation of water/TBA and water/TMAO mixtures have been studied at a cosolvent mole fraction, x2=0.02. Water perturbation induced by TBA and TMAO molecules has been studied also at x2=0.02 and 0.05 by using the absorption bands due to the vibrational modes of water in the infrared region. The data show, in agreement with molecular dynamics simulation results, that TMAO and TBA interaction with water are remarkably different and that water molecules are more tightly coordinated by TMAO than by TBA.