A nonlinear extended Kalman filter has been developed to infer the outlet composition of a ternary distillation column from temperature measurements. The reliability of the developed estimator is discussed with respect to disturbances on either steam, reflux and feed flowrates. It is shown that the accuracy of the adopted vapor-liquid equilibrium model plays a crucial role. The performance of the estimator is tested by comparison with actual outlet compositions measured in a pilot plant, where the separation of a ternary mixture of ethanol, tert-butanol and water is carried out.