The thermodynamic parameters such as partial molar enthalpy (Delta H-H), partial molar entropy (Delta S-H), and partial molar excess entropy (Delta S-H(E)) of the dissolved hydrogen in the hydrogen storage unit ST-90(R) containing 18.6 kg of misch metal based AB(5) alloy are evaluated based on the calculation of Sievert＇s constant and from Clausius-Clapeyron equation. The partial molar enthalpy at infinite dilution (Delta H-H(0)) and the partial molar excess entropy at infinite dilution (Delta S-H(E.0)) have been obtained from the dependence of and on the hydrogen concentration. The nature of metal-hydrogen and hydrogen interactions are also evaluated for the three single phases namely alpha, beta and gamma. The hydrogen-hydrogen interaction is attractive in the alpha phase and repulsive in the beta and gamma phases. For the mixed phases alpha + beta and beta + gamma, it is very difficult to predict the nature of the interactions either as attractive or repulsive.