To evaluate the molecular weight distribution in a reversible case, the author has considered a statistically averaged assemblage of all the possible structures of polymers of the same weight, a "statistical polymer". This is characterized by the following parameters : the number of units on some level (presence), the ability to accept new units (vacancies), and the number of units which have only one bond with the polymer. The polymerization has been considered as a reaction between a vacancy and a monomer, while the destruction has been considered as a decomposition of the bond between the basis polymer and an extreme unit. The final equation for the equilibrium constant contains the total numbers of vacancies and of extreme units.