We perform expanded ensemble Monte Carlo simulations in order to directly calculate the chemical potentials of carbon dioxide as solvent and those of hydrocarbons as solutes.Recently developed adaptive method is employed to find weight factors during the simulation,which is crucial to achieving high accuracy for free energy calculation.The present simulation method enables us to obtain chemical potentials of large solute molecules dissolved into a compressed CO2 phase from a single run of simulation.Simulation results for the excess chemical potentials of pure carbon dioxide at 300,325 and 350 K are compared with experimental data and with the predictions of Peng-Robinson equation of state.A good agreement is found for high pressures up to 500 bar.The chemical potentials of hydrocarbon solutes dissolved in carbon dioxide at infinite dilution are predicted by simulation without adjustable binary parameter.Less than ten intermediate states are necessary to insert (or delete) hydrocarbon solute molecules into the dense CO2 phase of 1.0 g/cm3.