Abstract: In literature, assumptions of negligible holdup or constant molal holdup are usually adopted for the batch rectification calculations. It is, however, that in actual cases the number of moles of holdup will vary from plate to plate and from time to time as composition does; therefore, the error introduced when using the constant molal holdup assumption will be con-siderable to a large amount of mixtures. In the present paper the assump-tion of constant volume holdup is proposed and therefrom equations are derived for the following processes. At the end of the "prerun" period, the amount and composition of Liquid in the still, we and Xwe, may be calculated by solving the following equations: where "h" denotes the volume of liquid holdup for each plate and "H" the corresponding number of moles, "a" and "b" are defined by equation (1) and may be considered as constants, "W0" is the moles of liquid origin-ally charged and "N" is the total number of plates of the column. For constant distillate composition processes, where "D" denotes moles of distillate and Y" is the yield fraction. By introducing the concepts of fictitious straight equilibrium line and fictitious straight operating line, the following equations are obtained: where "m" and "p" are the slopes of fictitious equilibrium line and operat-ing line respectively. From equations (6) and (9) "p" and "mD" can be solved, substitute "p" into equations (7) and (8) "mn" and "xn" are obtain-ed, then from values of "xn" and equations (4) and (5) D, W, and Y are determined. For intermittent processes with each withdrawal at steady state of the column, where V= volume of withdrawal, subscripts "q" and "q - 1" denote q-th and (q-1)-th round of steady state in the column respectively. Combine equations (10) and (11) with equation (3), successive values of "WE" and "XWe" can be obtained. The yield fraction after the s-th round of withdrawal is where For non-ideal systems, unless the equilibrium curve can be represented by y = Ax + Bx2 + Cx3, the same procedure outlined above can be followed but with instead of equations (8) and (9).