Reaction and phase equilibria in the isobutene + methanol + MTBE ternary system were studied using the reaction-ensemble Monte Carlo (REMC) simulation method. The system was modeled at the molecular level by an OPLS force field. No adjustable binary cross-interaction parameters or mixture data of any kind were used in the simulation model, and only vapor-pressure data for the pure components was required as input. The REMC method also computes excess internal energies and molar volumes as a biproduct of the simulations. Both the nonreacting and reacting ternary systems were considered over the temperature range of practical interest at 5 bar. Results are compared with the calculations using two conventional thermodynamic approaches: the Wilson and UNIFAC free-energy models for the liquid phase, together with a truncated virial equation of state for the gas phase in both cases. Computer simulation results were similar to those of the thermodynamic approaches, and they are arguably more accurate.