In the polymerization of caprolactam in presence of 6一amino—caproic acid and benzoic acid．experimental results indicated that the r61e played by the benzoic acid in-cluded：the formation of polymer chains by“induction”，the acid—splitting reaction of the macromolecular chains and the acylation reaction of the amino-group of the macro-molecules．During the beginning stage of the reaction，benzoic acid exerted a catalytic action on the monomer molecules causing them to unite to macromolecular chains and did not add to the monomer．As the reaction was going on，when the number of macromole-cules gradually increased，acid splitting reaction became more and more pronounced；and，after 3—4 hours’ reaction， it surpassed p01ycondensation and made the molecular weight of the polymer decreased and the carboxyl．content increased．The acylation reaction was clearly indicated after 7—8 hours’reaction when the quantity of the polymer formed and the molecular weight of the polymer have already reached constant values，while the consumption of the benzoic acid was still going on．At the end of the reaction．when the ultimate equilibrium has been established，there was still found free benzoic acid and free carboxyl-groups；this indicated that acid-splitting， acylation and polycondensation are all reversible reactions．In the experiments on the polymerization of caprolactam in presense of water but in absense of carboxylic acid，the polymers obtained was heated'at 250°。270°or at 300°under ordinary pressure while a stream of nitrogen was passed through．The molecular weight of the polymer did not increase after heated for 6 hours (and in case of 250°，for 15 hours)．The reason for no further condensation might be that the reaction medium became highly viscous which prevented the escape of water molecules and made the dif-fusion and collision of the macromolecules difficult． At the sametime，the steric hinde-rance of the macromolecular chain exerted on the functional groups became more and more pronounced when the chain reached certain definite 1ength．These all made the polycondensation stopped at a certain stage．