Reactivities of amines and reaction regions in aminolysis of PET fibers were investigated by using various primary amines, ammonia, and hydrazine. Aminolysis occurs randomly and the reaction rate is decreased in the order of MA>HH>EA>n-PA>n-BA>EDA>AM>MEA>iso-BA>sec-BA. There is no correlation between pKa values of amines and aminolysis rates, however, hydrazinolysis rate of PET fibers is greater than inferred from pKa value of hydrazine. Relative phenol adsorption by aminolyzed PET fibers is linearly correlated with the degree of degradation. In giving the above correlation, amines are classified into two groups, i.e. EDA, MEA and MA, EA, AM, HH. Amines in the same group seem to attack similar regions of PET fibers.Three groups of amines give different alkaline hydrolysis behaviors of aminolyzed PET fibers; group 1 is MA, EA, n-PA, n-BA, group 2 is EDA, MEA and group 3 is AM, HH. Alkaline hydrolysis behaviors of PET fibers degraded by amines in group 1 is as same with untreated PET fibers. Accordingly it may be supposed that aminolysis proceeded uniformly in the fibers. Remarkable weight loss is observed in the early stage of alkaline hydrolysis of PET fibers degraded by amines in group 2, but after that stage the hydrolysis behaviors are similar to that of the group 1. In this case intrinsic viscosity number has increased in the early stage of the hydrolysis. This effect seems to be due to the formation of oligo-esteramides near surfaces of the fibers. Alkaline hydrolysis rates of PET fibers treated by AM and HH increased gradually, as reported in detail in a previous paper. Reactivities of AM and HH are different from other amines, and reactions seem to occur in restricted regions in the fiber.