Using the recursive equation error method (REEM), the compositions of ternary flash drum have been estimated from the temperature of the drum and the product flow rates. If the equilibrium relationship is accurately known, the composition of the multicomponent system can be inferred quite reasonably. Compared with the standard recursive prediction error method (RPEM), REEM provides comparable estimation white the calculation is much simple and fast. (C) Elsevier Science Ltd. All rights reserved.