The three theoretical molecular weight distribution functions——the Flory．Poisson and Korshak distributions------are analysed and compared on the basis of the dependence of the width index xw/xn on the extent of reaction p．The logical error associated with the Korshak distribution is readily shown． After removal of monomer and oligomers from a polycondensation system，which obey the Flory distribution，the width of the mole-cular weight distribution of the resulting polymer is much lower than the theoretical width of the Flory distribution．and is close to that of the Poisson distribution．