Molecular dynamics simulations have been performed for water-tertiary butyl alcohol (TBA) mixtures in the water rich region. Examination of the Kirkwood-Buff integrals, local composition, and potential mean force for concentration in the range 0.05-0.07 TBA mole fraction leads to insight into the unexpected behaviors of some thermodynamics properties. Hydrophobic hydration phenomena and solvent-solute association are discussed at the molecular level. Since the hydrogen bond network elasticity modulus is a quantitative measure of the resistance of the water hydrogen bonds network to external perturbation arising from solvent-solute interactions, a first principle calculation of the elasticity modulus was carried out. (C) 2003 Published by Elsevier B.V.