PURPOSE: To obtain the titled compound economically advantageously in high yield, useful as an intermediate for guazatine, a fungicide for agriculture and gardening, by depolymerizing polyoctamethylenepolyamines under heating in the presence of ammonium sulfonate, ammonia, and water.

CONSTITUTION: In preparing dioctamethylenetriamine, an intermediate for guazatine, by dimerization of octamethylenediamine through removal of ammonia, nonvolatile high polymer polyoctamethylenepolyamines which are trimer or highen polymer prepared in a large amount as by-products are depolymerized in the presence of an ammonium salt of a sulfonic acid shown by the formula RSO3H (R is 1W5C alkyl, phenyl, or 1W5C alkyl-substituted phenyl), ammonia, and water at 200W300°C, and octamethylenediamine and octamethylenetriamine are recovered. In the operation, 0.5W5mol ammonium sulfonate and 5W20mol ammonia are used based on 1 nitrogen atom in the polyoctamethylenepolyamines.