The effects of chain-length of the diols and aliphatic dicarboxylic acids on the rate of polyesterification were studied. It was shown that the rate is only slightly influenced by the chain-length of the dicarboxylic acid. On the contrary, the reaction rate increases considerably with the chainlength of the diols. The etheric diols are exceptions since they produce lower rates in polyesterification than the diols with pure hydrocarbon chains of similar length. For the dicarboxylic acids, the results are interpreted as follows: during the polyesterification there are two opposite effects of the chainlength. Firstly, the direct esterification rate decreases with the increasing chain-length due to the reduction in the proton concentration; secondly, the rate of the simultaneous reverse reaction also decreases and the over-aU rate of esterification still increases. The chain-length of the diol, however, influences significantly only the reverse reaction. Its effect on the over-aU reaction rate is, therefore, greater than the resultant of the two opposing effects of chain-length of the acid. The influences on the reverse reaction are connected with the susceptibility to hydrolysis, first as hydrolytic susceptibilities of polyesters from diols containing shorter chain or etheric oxygen are higher. !