The separation performance of the metal - organic framework MIL-47 for xylene isomers was studied by Monte Carlo simulations in the grand-canonical ensemble. The experimental adsorption isotherms of xylene isomers in MIL-47 between 343 and 423 K were reproduced by using force fields available in the literature. Mixture isotherms were computed and compared with the mixture isotherms predicted by using pure component adsorption data and the ideal adsorption solution theory. This theory accurately predicts mixture isotherms of xylene isomers in MIL-47. the ideal adsorption sorption theory was further employed to calculate the separation factors of xylene isomers in MIL-47. The order of preferential adsorption was found to be ortho > para > meta. The adsorption selectivity was found to increase with pressure, and the results showed a good agreement with the experimental data available. Henry coefficients and low coverage heats of adsorption and adsorption entropies were computed at 543 K showing an excellent agreement with experiments. It was found that the reason for adsorption selectivity is the interaction of CH3 groups of neighboring molecules at high loadings, mainly for molecules adsorbed on the same wall of the MIL-47 channels.