A batch distillation of a crude tetraethoxysilane containing 8mol% triethoxysilane was performed in a glass packed column, 2.54 cm in diameter and 1m in height. Two distillate rates, 3.0mL/min and 6.0mL/min, were used and the reflux ratio was varied up to 3.0. Experimental data were compared with predicted values by Pro/II, a process simulator widely used in the chemical industry. The differential condensation of the vapor in the packed column due to heat losses from the vapor to the column internals and to the surroundings affected the separation efficiency seriously so that a considerable discrepancy was observed between experimental data and prediction by Pro/II in which such heat-loss effects are unaccountable. A model was developed to explain the effect of the differential condensation. For a larger distillation unit scaled up by 100 times where the heat-loss effect is regarded to be minimal, Pro/II simulations were performed to produce 99.9% TEOS with varying reflux ratio, number of stage, and feed composition.