Abstract: In recent years the Monte Carlo sampling technique has proved its value for calculating the thermodynamic properties of model fluids and solids. In this paper, the osmotic pressures are calculated by Metropolis Monte Carlo techniques. The activity coefficients of solvents in various TBP-diluent systems are obtained.The results are in agreement with experimental data and other theoretical values.A special tech nique is developed which improves the convergence of the calculation considerably.